WEST HAVEN, CONN. -- It has been 25 years since Michael M. Plaskon was a Marine infantry machine gunner in Vietnam. But when the United States went to war in the Persian Gulf, he was overwhelmed by painful flashbacks of combat, which led the 44-year-old veteran to be hospitalized again at the Department of Veterans Affairs Medical Center here.

The 2-year-old program, one of four sites of the VA's National Center for Post-Traumatic Stress Disorder (PTSD), is in the unusual position of treating the psychiatric victims of the last major war, such as Plaskon, while preparing for new casualties from the current one.

"Never before have we been so prepared. Our treatment system is ready," said David Read Johnson, a clinical psychologist who directs the 15-bed PTSD inpatient unit here. The West Haven PTSD program, affiliated with Yale University School of Medicine, currently treats more than 200 PTSD patients and is a major research center on post-traumatic stress disorder.

Since the Gulf war began, Johnson said, as many as 10 percent of PTSD patients treated at West Haven have experienced relapses, including suicide attempts, alcoholism, depression or flashbacks.

"We're hoping to prevent the kinds of chronic PTSD experienced after the Vietnam War," said Larry Lehmann, associate director for psychiatry at VA headquarters in Washington. Designated military and veterans' hospitals around the country are now on alert for Persian Gulf psychiatric casualties, particularly in the event of a major ground war.

Victims of post-traumatic stress disorder have often experienced or witnessed a life-threatening event that they persistently re-experience. Classic manifestations include flashbacks or terrifying nightmares; an emotional numbing or avoidance of the outside world and any reminders of the trauma; trouble sleeping or concentrating, a highly aroused emotional state and uncontrollable anger.

A study ordered by Congress and conducted by the Research Triangle Institute, a North Carolina nonprofit research organization, found that as of 1988 about 480,000 Vietnam veterans -- 15 percent of those who served -- were still suffering from the disorder. Approximately 30 percent -- about 960,000 veterans -- have experienced post-traumatic stress at some time since the war, the study found.

Most of the research on the controversial disorder is still relatively new and involves the most severe cases among Vietnam veterans. Experts say that the length of the war and its unpopularity may have exacerbated the initial trauma.

There is no way to predict how many of the 505,000 U.S. troops stationed in the Persian Gulf will suffer post-traumatic stress. But it is clear that so far, in contrast to Vietnam, the Gulf conflict enjoys widespread popular support. Many war protesters stress that while they oppose the war, they support the troops, which was not the case during Vietnam.

To minimize the impact and extent of post-traumatic stress, all Gulf soldiers sent to West Haven will automatically undergo psychiatric screening and evaluation, according to Steven M. Southwick, director of the PTSD program. "That's new," he said. "Usually, psychiatrists are called in only after there's a real problem." Those deemed in need of help will receive therapy as well as peer counseling from Vietnam veterans who have suffered from post-traumatic stress.

Although military officials say they are taking special pains to treat the disorder among Gulf troops, the phenomenon of wartime trauma is not new. Some Civil War soldiers were said to have suffered from "soldier's heart," while World War I veterans experienced "shell shock."

Psychiatrists recognized "combat neurosis" and "gross stress reaction" in World War II veterans. But it was not until 1980, prodded by victims of "post-Vietnam syndrome," that the American Psychiatric Association added "post-traumatic stress disorder" to its official diagnostic manual.

Since then, recognition of the disorder -- and some critics say overuse of the diagnosis -- has increased dramatically. PTSD also has been diagnosed among survivors of civilian traumas, ranging from natural disasters such as earthquakes to plane crashes and violent crimes such as rape.

"Two to 9 percent of adults may have had the illness," said Jonathan Davidson, a Duke University psychiatrist who is co-chair of an APA committee that is considering how broadly to define post-traumatic stress disorder. "Physical injury or the clear threat of death seem most likely to set it off," says Davidson.

Trouble in personal relationships, difficulty holding a job and abuse of alcohol or drugs often accompany post-traumatic stress. Although symptoms may appear soon after the event and persist, sometimes they don't surface for months or even years afterward.

"After Vietnam, the single strongest predictor of who got PTSD was the experiences they had while in the war," said William E. Schlenger, a psychologist who directs mental and behavioral health research for Research Triangle Institute. The most common factors were the amount of combat, especially ground fighting, and exposure to death and to abusive violence, particularly acts of torture.

Schlenger said that while certain factors, such as a family history of psychiatric disorders or domestic violence, may make a person more vulnerable, those experiences appear to play a minor role compared to the trauma of war.

Increasingly, researchers say, they are able to document physiological changes that occur in people with severe cases. "When you are under severe stress, the body needs to respond to get out of danger . . . Reflexes quicken and the body assumes a 'fight or flight' readiness," said Dennis S. Charney, chief of psychiatry at the West Haven VA and director of the national PTSD Center's clinical neuroscience division.

Some studies have suggested that victims may have chronically abnormal levels of certain hormones and other chemicals, including adrenaline, cortisol and natural painkillers, as well as higher blood pressure and heart rate in response to the most stressful reminders of war.

"I believe that many of the symptoms of PTSD are related to biochemical changes in brain function. We have evidence demonstrating this link," said West Haven neuroscience researcher John H. Krystal.

Better understanding of the biology of post-traumatic stress will ultimately help researchers design drugs and other therapies to treat the disorder. Current treatment includes drugs to treat anxiety or depression, individual and group psychotherapy and self-help programs for patients and their families.

"One of the most positive signs today is the recognition that psychological casualties are part of what we have to expect," said Susan D. Solomon, chief of violence and traumatic stress research at the National Institute of Mental Health. "That was certainly not characteristic of Vietnam, when there was large-scale denial. If there is more understanding that these responses are normal, it takes a lot of the stigma away and makes it easier to recover."