The federal government is trying to have things two ways in a toxic chemical issue that involves billions of dollars and countless human lives.

In two court cases, the government is arguing that the pesticide pollutant called dioxin has been proven so deadly that all polluted pesticide use should be halted and that wastes containing dioxin should be banned.

In other court cases, the government is arguing that the same evidence is inconclusive and does not prove anything about the human effects of a low dose.

No one denies that dioxin is deadly in the large doses that have taken in industrial accidents. But the Environmental Protection Ageency has decided that even very low doses are dangerous. The Department of Defense and the Veterans Administration, looking at the same evidence, are not convinced.

It is not the first time the government has been of two minds on a single issue. It may be among the first examples of government attorneys arguing both sides simultaneously.

At stake are possibly billions of dollars in compensation and treatment for hundreds of Vietnam veterans who claim that the dioxin in Agent Orange, a powerful defoliant, caused them serious health problems. Also at stake are the $12 million that Dow Chemical Corp., gets in annual sales of 2,4.5T, a herbicide that was part of Agent Orange and the futures of several chemical companies making it now.

The EPA opened its case this week in Little Rock, Ark., against Vertac Chemical Corp., asking a federal judge to prohibit the Memphis firm from dumping any more wastes containing dioxin at its Jacksonville, Ark., plant.

"It's one of the most toxic substances known to man, measured in parts per trillion," said James W. Moorman, assistant U.S. attorney general for lands and natural resources. "This is the first case we've brought involving dioxin and so it's very important to us."

Dioxin is an unavoidable contaminant of the herbicide 2,4,5-T that has been produced by Vertac since 1956. The EPA banned most uses of 2,4,5-T last year after a group of women in Alsea, Ore., developed high rates of miscarriage after the spraying of nearby forest land. The Army stopped using Agent Orange in Vietnam after high rates of stillbirths and miscarriages were reported among sprayed Vietnamese.

In another proceeding, EPA is asking that all remaining used of 2,4,5-T be halted by an administrative law judge. That decision is two years away, but EPA's position is consistent:

"We have not been able to establish a no-effect level," one that produces no tissue change in laboratory animals, said Paul Lapsley of EPA's pesticides and toxics office. "There is reason to be concerned about exposure at any level."

But over at the Veterans Administration, the same studies EPA is citing in court have been ruled inconclusive.

"The jury is still out," VA Adiminstrator Max Cleland told February congressional hearings on Agent Orange. Inan interview, he said studies showing dioxin is carcinogenic are one thing but proof still is lacking that "brushing up against a leaf in Vietnam" can cause the ailments that veterans allege: liver troubles, impotence, headaches, cancers, blackouts, and birth defects in their offspring.

The only conclusive evidence involves a skin disorder called chloracne, and the VA readily pays benefits to any veteran who suffers from that, Cleland said.

Dr. Barclay Shepherd, VA coordinator of medicine and surgery for Agent Orange, cited problems in the studies upon which EPA relies. Some involve higher or longer exposure levels than veterans experienced; others have statistical problems or ignore the decay that dioxin undergoes in sunlight, he said. Humans appear to have more resistance than laboratory animals.

"EPA and VA have different missions," Cleland said. "One says we must be very cautious in the future . . . they can go with it based on far more limited evidence. They don't have to start paying compensation based on scientific studies like I do."

Much the same arguments have been made by Dow Chemical in defending 2,4,5-T from the proposed ban and by Vertac Chemical in defending its Jacksonville plant.

"We don't think there's any hazard when it's properly used or we wouldn't be selling it," a Dow spokesman said.

Allan Gates, attorney for Vertac, said the amounts of dioxin found around the plant site are "infinitely small compared to the current tolerance level," that EPA allows where 2,4,5-T use is still permitted. "There's no question [dioxin] is toxic at high exposures but the question here is whether there are those high levels," he added.

Steve Cahmplin of the Vietnam Veterans of America, which is pressing the VA in lawsuits to change its mind, said the government split "is approaching a policy crisis.

"The VA has no justification for a policy it now holds nearly in isolation from the rest of the federal government," he added.

The validity of the various studies is the main focus of the court battles.

Meanwhile, the EPA was busy last week in Aurora, Mo., digging up a trench full of 55-gallon drums believed to contain dioxin-laden wastes from a defunct chemical company's hexachlorophene production. Farmer James Denney told the EPA that Northeast Pharmaceutical and Chemical co. paid him $150 in 1970 to allow the burial of six truckloads of the drums and told him to keep his mouth shut.

In Washington, the EPA asked Dow Chemical to provide more information on 2,4-D, the pesticide that made up the other half of Agent Orange and is now the most widely used in the country. Although no dioxin has been found in 2,4,-D, which goes into 1,500 products at the rate of 70 million pounds a year, the EPA said studies on its cancer-causing possibilities are inconclusive.

Failure to provide information in 90 days could mean a ban on 2,4-D, but Dow said it expected no problems.

A House subcommittee has approved a measure to help clean up orphan waste dumps through a $600 million "superfund," financed 50-50 by government and industry. The Chemical Manufacturers Association opposes the idea on grounds it unconstitutionally assesses them for sins of past firms, while ecologists protest that the measure has been gutted from original, much larger proposals.

And in Kalamazoo, Mich., a jury acquited a Vietnam veteran of drunken driving when he said his erratic maneuvering resulted from his exposure to Agent Orange. Dennis Briggs, 30, of Mendon, Mich., has lost his driver's license for refusing to submit to a breathalyzer test, but he said Agent Orange exposure was his only defense. It was the truth."