Twelve Americans roll through the Vietnam countryside as part of a medical experiment.They are the first of what may become successive waves of veterans whose therapy is to go back to the war to get over it. The goal is to relieve the suffering of thousands of Vietnam veterans afflicted with the lingering stress of war's trauma. Treatment begins where the nightmares originated.

Cramped in a bus, the 12 veterans, including two nurses, were arguing over whether they should swing off Route One and visit the village of Mylai, where U.S. soldiers in 1968 had herded more than 100 South Vietnamese men, women and children into a ditch and shot them to death.

"I voted against it," recalled Donald DePina, 41, of New Bedford, Mass., a former machine gunner whose Americal Division had committed the atrocity. Even though he was not at Mylai at the time, "I was afraid of being haunted by what I would see. I had been trying to forget the war for 20 years."

DePina lost. The bus rolled on, its American flag still displayed. Then, just before Mylai, the bus had to stop for a funeral procession blocking the road. The slowly marching mourners stared up at the Americans.

"I was very apprehensive," said formr Navy medic Michael House, 42, who was among those interviewed by The Post after the trip. "I was afraid they'd take us out of the bus and parade us through the village as war criminals."

The Vietnamese interpreter got out of the bus and explained to the elders in the funeral procession that the Americans would like to go to the center of Mylai. The crowd moved off to the side of the road.

Soon DePina, House and the others found themselves inside the small building housing memorials to the Mylai victims. The Vietnamese woman in charge of the memorial told her visitors that American forces had killed 504 Mylai villagers on March 16, 1968. The U.S. Army had accused Lt. William L. Calley Jr. of murdering 102 villagers that day. "I will show you," she said, and led the way to the paths outside.

She took the Americans to the ditch where the massacre occurred. She explained that the victims, including many pregnant women, were buried where they fell. Plaques marked the spaced-apart graves. As the Americans moved slowly from grave to grave in Mylai and viewed the murals depicting the shooting by U.S. riflemen and strafing by aircraft, a crowd of villagers began to form, following the strangers in their village step by step.

"I looked back," DePina said, "and saw that all the children were smiling at me. I started crying.

"Back inside the building," the former machine gunner recalled, "our guide asked us to sign the visitor's book. I wrote: 'Please forgive us.' Then I signed my name and put down Americal Division."

Toward the end of the visit, three women from Mylai served the visitors tea and smiled at them. The woman who led the tour told the veterans: "The past is past. We know the American people did not want this to happen. It is time to forget and become friends."

Ronald Szpond, 44, a onetime Marine sniper, asked the Vietnamese guide if he could pick a single marigold from the memorial garden at Mylai. She consented. Szpond had plans for that marigold when the bus moved north to his old battlegrounds around the former American base at Chulai.

Back aboard the bus there was a solemn quiet. Almost everybody was crying. Jaime Rodriguez, a former Army rifleman who thought up the idea of taking a group of veterans back to Vietnam, recalled that "finally, somebody said, 'Here the South hasn't forgiven the North for something that happened 100 years ago, and these people forgive us for something that happened only 20 years ago.' "

Rodriguez is research coordinator at the William Joiner Center at the University of Massachusetts in Boston. The center, staffed largely by Vietnam veterans, sponsors counseling and other activities designed to help Vietnam vets overcome trauma generated by the war. Rodriguez said he returned to his old Vietnam battlefields in 1988 and found the experience so uplifting and curative for his own trauma that he pressed the center to sponsor such trips for other veterans.

The result is a project called Full Circle, designed to help those suffering post-traumatic stress disorder -- PTSD. Manifestations of PTSD range from nightmares, to alcoholism, to guilt, to anger to violent behavior.

The Joiner Center, with the help of psychologists who specialize in PTSD, is screening veterans who want to go back to Vietnam and are willing to be tested before and after their trip so that the impact of returning to the scene of trauma can be clinically measured.

Kevin Bowen and David Hunt, co-directors of the Joiner Center, said that this return program is unique because the results are being carefully calibrated in hopes of reaching a medical judgment about whether such intervention helps or hurts veterans struggling to forget the war and get on with their lives.

DePina, House and Szpond were part of the first squad of veterans, most of whom work at veterans' centers in Massachusetts, who went to Vietnam under this clinical experiment last month. In addition to Mylai, they went to battlefields where they had fought and to Ho Chi Minh City (Saigon) and Hanoi in a three-week tour of what had been South and North Vietnam when the U.S. government sent them to Indo-China in the 1960s. The Joiner Center intends to sponsor other trips in the near future. "What's this all for?" Jessica Wolfe, a PTSD psychologist at the Boston Veterans Medical Center and consultant on the Full Circle experiment, asked rhetorically? "It's to help improve the recognition and treatment of PTSD."

Terence M. Keane, director of psychology at Boston Veterans Medical Center, said his fondest hope for Full Circle is that "we find the trip had very positive effects on the lives of the people who went" and that those effects can be measured and presented to the world health community with such conclusiveness that many other veterans will be given the opportunity to return to Vietnam.

Wolfe and Keane served as psychologists for Full Circle in their non-government capacity as partners in the Traumatic Stress Resources consulting firm. They interviewed, tested and screened Massachusetts veterans who volunteered to go back to Vietnam to determine if they were stable enough and left the final selection up to the administrators of the Joiner Center. Wolfe said they will re-interview and re-test the 12 who just returned from Vietnam and repeat the examinations in six months and again at the end of a year to determine if there have been lasting, measurable benefits.

"This is not for everybody," said Wolfe of going back. "This is not a cruise. It's an intervention. It probably has some re-exposure to previous trauma." This could be destabilizing for some veterans suffering from PTSD.

At this early point in the study, "we're not interested in taking people from wards here and bringing them back to Vietnam," Keane said. For one thing, they said, this would require sending a supporting team of mental health specialists to Vietnam to treat veterans who could not handle the stress of the return. For another, said officials at the Joiner Center, funds for the trips are limited and Vietnam does not have the facilities to handle large groups of veterans. Under the first phase of Full Circle, the Joiner Center financed about two thirds of the $3,000 in transportation and living expenses and the veterans came up with the rest.

"Nothing else has done much good to cure veterans of their PTSD," said Joiner Center co-director Bowen. "This is a program thought up by Vietnam veterans for Vietnam veterans that offers some real hope."

Arguments continue over how many Vietnam veterans are suffering from PTSD and whether their problems in adjusting to civilian life were caused by their wartime service. A comprehensive study conducted by the Research Triangle Institute of Research Triangle, N.C., concluded that PTSD was widespread. The institute's 1988 report entitled "National Vietnam Veterans Readjustment Study" said that:

479,000 of the 3.14 million men who served in Vietnam currently are afflicted with PTSD.

610 of the 7,200 women who served in Vietnam suffer from PTSD.

Over the course of their lifetime, 30.9 percent of the males and 26.9 percent of the females who served in Vietnam will have suffered some form of PTSD.

Among male racial groups who served in Vietnam and are currently afflicted with PTSD, 27.9 percent are Hispanics, 20.6 percent blacks and 13.7 percent whites.

PTSD specialists contend that two reasons so many Vietnam veterans have suffered from post-combat stress is that few were warmly welcomed home by their countrymen and, until recently, their government would not permit them to return to former battlefields, as thousands of earlier veterans did shortly after World War I and World War II.

Szpond, the former sniper who picked the marigold at Mylai, said he had killed from 12 to 24 enemy soldiers by himself. He said it was years before he was willing to admit that his alcohol and drug addictions stem from his feeling of guilt. He ultimately sought help from the VA.

"They told me I had to find a way to forgive myself for killing those people as a sniper; that my subconscious was fighting my conscious." He said as a young Marine sniper in Vietnam in 1965 that he was proud of his record of 12 positive and 12 probable kills. "But the warrior's pride turned into the warrior's guilt," he said.

"I was raised as a Catholic," Szpond continued. "Altar boy; sang in the choir -- the whole bit. It was difficult for me to go over there at a ripe young age and start killing people. I had been taught it was wrong to kill. But everybody said it was okay to kill in wartime.

"The killing became very personal for me when I became one of the Marines' 12 snipers in Vietnam from November of 1965 through April of 1966. I'd go out on patrols with different outfits. When they wanted somebody killed, they'd say, 'Sniper up!'

"When I squeezed that round off and the warrior fell, I knew I had done it. Nobody else. There was no question I had killed as a sniper. A firefight is different. You don't know who killed the guy. I was proud back then of being a Marine sniper. I have no problem with patriotism. I went there do a job."

Szpond was wounded a second time and left Vietnam in April 1966. He said he became a New Jersey state trooper and then a policeman in Elizabeth, N.J. His PTSD didn't become evident until 1980. He said he would go to a train wreck and flash back to traumatic scenes in Vietnam. "I knew something had happened to me when I had a robber who shot a mailman cornered, a cop's dream, but I couldn't shoot the guy."

Szpond said he started drinking heavily in the early 1980s in hopes of dulling the pain that came from his recurring nightmares about Vietnam. According to doctors, nightmares are a common manifestation of PTSD. "I knew the beast was within me. I became cross-addicted, alcohol and codeine."

He retired from the Elizabeth police force in 1985, remarried and moved to Falmouth, Mass., on Cape Cod. But the nightmares about Vietnam persisted, one in particular: "I'm pinned up high, like being crucified, and I'm looking down at Vietcong and North Vietnamese uniforms," Szpond said. "I can't see their faces, but I can see them thrusting up at me. Each time they thrust, I feel this excruciating pain in my old war wounds. I wet the bed and wake up sweating and trembling."

When the group got to his old base at Chulai, the gates were locked. "I looked around and saw this Vietnamese war memorial on top of a hill," said Szpond. "I climbed the hill with the marigold. I had to find a way to forgive myself. I had already paid respect to my brother warriors at The Wall in Washington. I had to pay my respects to those people I had killed.

"I put the marigold on the Vietnamese war memorial on that hill and said a prayer. I said: 'You are my Savior; my Redeemer and my Lord. Sacred heart of the crucified Jesus, help me to forgive myself so I may forgive others.'

"Shortly thereafter," Szpond continued, "I felt this comfort of having whatever it was lifted. I felt it within myself. I felt I was forgiven by the same ghost that was haunting me all these years."

The two nurses on the trip, Martha Green, 44, of Hinsdale, Mass., and Judy Tracy, 43, of Brookline, Mass., said they had similar experiences by returning to the areas of Vietnam where they had treated so many horrible war wounds. Until they went back to Vietnam, they said, the images of Vietnam they could not get out of their minds were ones of blood, helicopters, explosions, smoke, anguish, pain, fear and hatred.

Today, the Soviets control Camranh Bay and the base at Chulai is closed down so that Green, a former Army lieutenant, could not return to the hospital where she had served as a nurse in 1968 and 1969. But everywhere she went in Vietnam this time, she said, "there was love shown to us. I didn't have an appreciation of the Vietnamese people. I was able to replace all the negative in my mind with the positive. We were met by Vietcong and North Vietnamese Army officers, and they were so gracious to us. These people don't have freedom, but they do have peace. I finally feel at peace. Going there put the ghosts to rest."

Tracy, who did two tours as a civilian nurse in Vietnam in 1969 and 1971, did return to Holy Family Hospital in Quangngai where she had treated so many wounded South Vietnamese civilians and cared for war orphans. "As soon as I found myself standing in front of the locked gates to the hospital," Tracy recalled, "I started to cry."

PTSD specialists say sights, sounds and smells remindful of a traumatic experience often trigger such tears. They say the release is therapeutic.

A South Vietnamese family, seeing Tracy standing before the locked gates, motioned her to come into the hospital through a different entrance and escorted her to a reception room. When the Holy Family administrators learned who Tracy was and what she had done during the war, she said she was treated like a celebrity and escorted to the room that was to be dedicated to her and other nurses who had ministered to civilians during the war.

"None of us got any credit from our own country for what we did over there," Tracy said in a line that could have fit comfortably in the script of the movie, "Born on the Fourth of July." She said she was overwhelmed by the depth and warmth of her welcome at the hospital where she had seen such suffering.

"I looked down from where I was sitting in the room to be dedicated to us," she said, "and realized it was the very spot where my bed had been. I started to cry again. I came back . . . this time with totally different images in my mind about Vietnam. Now I see smiling Vietnamese faces. Faces that lit up when we said we were Americans."

House, who said he became a medic for the Marines because "I just didn't think I could kill anybody," said he experienced emotions similar to Szpond's when he returned to the demilitarized zone where he was wounded in 1967 while vainly trying to save a dying Marine.

"I walked up the hill at Conthien and looked out. I thought of all the things I had lost there; the buddies I couldn't save. I remembered lying there wounded waiting for the MedEvac {helicopter} to come. I wasn't afraid of death. I just didn't want to die on some hill 12,000 miles from home.

"I felt strangely in touch with the spirits of friends I had lost as I stood on the hill this time. I picked up some dirt to bring back. I felt I had finished my tour. I had felt guilty about getting wounded and leaving the men who needed me. I wanted them to know I had come back. I shed a few tears. It was the end. I had come back. Now I can go down to The Wall {Vietnam Memorial} and know we are friends. When I went there before, it was a grieving process. Now I can go back to the wall and just say hello."

Gumersindo Gomez, 41, of Springfield, Mass., and David F. Pye, 43, of Manchester, Mass., discovered that they had both fought on Black Virgin Mountain at Tayninh during the war. Seeing that once fearsome, bunkered and highly contested mountain at peace proved a relief for both of them, they said.

"The mountain is still the same," said Gomez, "except the foliage is gone. I walked halfway up that mountain where so many of my friends had died. We were supposed to be guarding the Army engineers on that mountain one night in January 1967. The Vietcong slipped through us and killed every one of the engineers. About 12 of them.

"Coming down that mountain, it really hit me," Gomez said. "The tightness came back. The memories came back. I cried. I screamed at the mountain: 'I walked away from you twice. I'm walking away from you again. {Expletive} you, Mountain!'

"I'm crying. I'm hurting because I lost some good friends there. But I'm walking away. It's a release. Everything in my chest comes out. No more pain. No more being afraid. All those bad images go away -- the bunkers, the concertina wire, the shooting."

Pye said he underwent the same kind of transformation as he went up and then down the mountain. But the mountain got in one last lick when he twisted his ankle just before he reached the foot of it.

Still, once he was off the mountain, he was free of the old foreboding and fright, Pye said. He studied the terrain with new eyes, and then he saw a Vietnamese film crew using the once fearsome mountain as a backdrop for a home entertainment movie. This ended Dave Pye's long war, the former rifleman said.

Before former rifleman Pye went off to Vietnam this time, he told Kim Wilkins, a friend and associate director of the RAP counseling center in Beverly, Mass., that "last time nobody welcomed me home. This time I wanted a brass band."

Wilkins did more than that. She persuaded The Brassmen of Gloucester, Mass., to play -- free of charge -- a rousing welcome for Pye and fellow veterans when they returned to Boston's Logan Airport -- and she arranged for a crowd of well-wishers to be there to wave American flags and signs of "Welcome Home!"

What Is PTSD?

Post Traumatic Stress Disorder is a condition in which a searing event in the past, such as military combat or involvement in a murder or an auto accident, interferes with normal living in the present. Symptoms range from persistent nightmares to violent behavior.

A number of studies suggest that a larger percentage of Vietnam veterans suffer PTSD than did veterans of earlier wars. A 1983 survey of mental health professionals in Veterans Administration hospitals, for example, revealed that these specialists regard "Vietnam combatants as more seriously disturbed than combatants of other wars." One possible reason, psychologists say, is that society did not embrace Vietnam veterans on their return.

Erwin R. Parson, a former Army medic and psychologist specializing in PTSD who encouraged the Full Circle experiment of sending selected veterans back to Vietnam and scientifically assessing the impact on them, wrote in the International Journal of Group Psychotherapy in July 1988 that guilt appears to be at the heart of the PTSD affecting many veterans.

"Guilt is always present after killing in war," Parson wrote. But "unlike previous wars in which actions taken in battle 'in the line of duty' were shared by family, local community, the nation and the world community -- soldiers who fought in Vietnam bear the full responsibility of conscience alone. For some veterans, this feeling of aloneness becomes unbearable . . .

"Post-traumatic guilt syndrome . . . is marked by a chronic life pattern of pervasive fears, self-defeating behavior, self-destructiveness, self-loathing, absence of self-caring skills and self-hate," according to Parson. The smell of diesel fuel, the sound of helicopters and the sight of troops can trigger fright and depression in Vietnam veterans, according to VA psychologists.

The Research Triangle Institute of Research Triangle Park, N.C., concluded in its "National Vietnam Veterans Readjustment Study" completed in 1988 that 15.2 percent -- about one in six -- of the 3.14 million men who served in Vietnam suffer from PTSD. At some time during their lives, the study said, 30.9 percent will experience PTSD.

Male Vietnam veterans with PTSD "are five times more likely than those without to be unemployed," the study reported, "and one in five has a history of extreme occupational instability."

Also, the study continued, 70 percent of PTSD veterans have been divorced, 35 percent of them two or more times; 55 percent "have high levels of problems" in being parents and 47.3 percent "report extreme levels of isolation from other people . . .