The pains in his side can come at any time. Sometimes they are sharp, knifing. At other times, they resemble a dull ache. They were worse when he drank heavily and regularly. But they didn't go away when he stopped.

John S. Crounse, 32, a pale, sad-eyed Vietnam veteran and University of Maryland student, doesn't know what causes the pain. Neither do the doctors. m

Crounse, who served as a naval adviser along the Mekong River in 1971, fears that his unexplained symptoms may be related to a chemical defoliant -- Agent Orange -- to which he and possibly 2.4 million servicemen were exposed in Vietnam, according to Verterans Administration estimates.

George T. Estry, 30, remembers the chemical came in 55-gallon drums banded with two orange stripes. It was dumped along the camp perimeter to keep down vegetation. He was a helicopter crew chief for the 101st Airborne Division and worked on helicopters that sprayed clouds of Agent Orange into dense jungles.

Estry, also a University of Maryland student who served in Vietnam from 1968 to 1970, complains of pains behind his eyes and unexplained fits of nervousness. He said doctors have been unable to explain his problems.

Joseph L. Beagle, 29, had two cysts removed from his back and suffers from gall bladder trouble. He served on helicopter gunships for the Army's 1st Cavalry Division in Vietnam and remembers something irritated the crew's eyes and noses. Crew members "couldn't get along without Vicks inhalers and eye drops," he said. He, too, thinks he was exposed to Agent Orange.

Crounse, Estry and Beagle were interviewed at the university's veterans club last week before a talk on Agent Orange and its possible effects by Frank McCarthy, president of AGent Orange Victims International (AOVI). The event, which attracted 40 people, was sponsored by the veterans club.

Agent Orange, widely used over a 10-year period in Vietnam to strip the jungle cover and destroy food crops, contained dioxin, a compound believed to be one of the most toxic chemicals ever developed and known to cause cancer and birth defects in animals.

Little is known about its effect on people. It is acknowledged to cause a painful, persistent skin rash called chloracne. But thousands of veterans claim it also causes cancer, liver damage, psychological and neurological disorders, miscarriages and a wide range of birth defects in their children.

McCarthy said though many servicemen did not have direct contact with Agent Orange, they may have been exposed. While the chemical decomposes after six to eight hours of direct sun-light, it was washed into groundwater, where it remained active, by heavy rains that fell daily during several months of the year in Vietnam. He said soldiers regularly drank the contaminated water from bomb craters and roadside ditches.

"We'd drop in the purification tablets, shake it up and wait 20 minutes. That's how we got poisoned," he said.

McCarthy, who served in Vietnam in 1965 and 1966, said he was exposed to Agent Orange and has suffered disabling headaches for 12 years.

AOVI was founded by Paul Rutershan, a vet who believed his stomach cancer was caused by his exposure to th dioxin in Agent Orange. His persistent claims in the national media that "I got killed in Vietnam and didn't even know it," drew the attention of thousands of veterans, many with unexplained symptoms, who believed they too might be victims. Rutershan died in December 1978.

The all-volunteer AVOI has nearly 10,000 members, according to McCarthy. It is devoted to drawing attention to veterans' claims and getting medical treatment and counseling for affected veterans.

Through its coordination, several thousand veterans have filed suit in federal court against five chemical companies which manufacture Agent Orange. The class action suit asks for a percentage of the companies' profits to establish a fund for treatment and research on the effects of Agent Orange.

To date, McCarthy said, the Veterans Administration acknowledges only chloracne as an Agent Orange-related disability that qualifies veterans for compensation.

Though VA hospitals will treat all veterans' ailments, vets can get no compensation for disabilities they believe are related to Agent Orange unless they can prove their symptoms were directly triggered by exposure to the chemical. So far, the VA has processed 1,651 claims of disability from or exposure to Agent Orange. Only two veterans have won compensation for chloracne.

A VA spokesman said the VA is processing thousands more claims. The VA is encouraging veterans to file claims of exposure to Agent Orange. If future studies show a connection between Agent Orange and physical disabilities, vets can then claim compensation back to the claim date.

McCarthy is bitter about what he charges is VA reluctance to face the issue. He believes the VA should warn all veterans that exposure may affect their health and the well-being of their off-spring, and should act as an advocate to get research on Agent Orange and treatment for contamination.

McCarthy said lack of information about Agent Orange has frightened many veterans and their families. One woman, he said, was determined to abort her unborn child because she feared the child would be born defective. Her husband had been exposed to the chemical.

"Vets are going around and getting vasectomies when they shouldn't," McCarthy said.

Several bills in Congress would provide benefits to veterans exposed to Agent Orange and to their birth defective children as well, McCarthy said. However, a spokesman for Rep. David E. Bonior (D-Mich.), who sponsored two such bills, said they have little chance of passing.

Marthina S. Cowart, a spokeswoman for the Veterans Administration, said while the VA "can't pay compensation on dioxin poisoning without medical evidence," it is "looking at every study that surfaces."

The Veterans Administration, HEW and the Department of Defense are part of a White House-appointed task force to gather research on Agent Orange.

Cowart said the VA has asked each branch of the armed services to provide it with complete information on troop movement and use of Agent Orange so it can compile a list of all servicemen who were exposed.

VA Administrator Max Cleland himself was exposed (to Agent Orange) "on two occasions," Cowart said, and is sensitive when veterans charge the VA is dragging its feet.

Some herbicides used domestically on crops and to clear rights of way for power lines and railroad tracks also contain dioxin. Last year the Environmental Protection Agency restricted their use while effects of dioxin are studied.

But increasingly, McCarthy said, the general population as well as veterans, are becoming concerned about the toxic effects of the chemical.

"It's not only a veterans' thing," said Mike Bjork, president of the veterans club. "It can affect everybody."