The large, windowless room, one flight above the Waterside Mall shopping center, was heavy yesterday with the sights of pinstripped lawyers surveying their cheerless home-away-from home for the next 18 months. The government's administrative law hearing on a proposed ban of the herbicide 2, 4, 5-T opened for its multimillion-dollar run.

At stake for Dow Chemical Co. are $12 million in annular sales of the weed killer plus a great deal of bad publicity and a possibly endless string of future lawsuits. For the Environmental Protection Agency, final prohibition of 2, 4, 5-T and its sister chemical, Silvex, would vindicate a decade of controversial research that says human lives are at risk.

The two sides will fight it out decorously amide the plastic paneling and the crumbling yellow vinyl chairs, be-reft of any pomp and circumstances, in that Washington bureaucratic marathon known as an administrative law hearing. This one is close to EPA headquarters but it could be anywhere in the city.

There are many of the trappings of a regular court trial, with sworn witnesses, rules of evidence, cross-examination and endless wrangling on procedure. Disputes over federal rules and regulators keep 1,100 administrative Social Security claims and personnel grievances for the most part, but also sitting in judgement for years at a time on complex regulatory cases like this one.

Dow lawyers had EPA attorneys outnumbered 10 to 6 on opening day, but one was already reading the newspapers while the others argued about admitting some evidence. It is going to be a long case.

No one expects the 40 or so spectators there yesterday to keep showing up very long. By the time Judge Edward B. Finch reaches his verdict in early 1982, Dow will have spent at least $1.5 million and EPA a couple of million more, and it won't be over then.

EPA administrator Douglas Costle will issue the final ruling on whether to ban all use of 2, 4, 5-T and Silvex, but that decision is certain to be taken to the U.S. Court of Appeals. A real decision is more than two years away.

EPA has gone through this procedure to ban 15 chemicals so far, and another 15 were taken off the market voluntarily by the manufacturers, according to EPA's assistant administrator for pesticides, Steven D. Jellinek. His office has 150 witnesses from around the world ready to testify that this pesticide should be banned.

On the other side, Dow has promised to invalidate most of the EPA studies and to present research of its own scientists that, as vice president and health director Etcyl Blair said, "shows this is truly a safe product, a very useful product for which there is a real need."

The EPA banned most uses of 2, 4, 5-T on a temporary emergency basis a year ago after women near Alsea, Ore., suffered a sharp rise in the rate of miscarriages following springtime spraying of forests near their homes.

Since then, the agency said in its pretrial brief, it has continued to receive "unprecedented and overwhelming" evidence that use of the pesticide causes reproductive problems, cancers and death among those exposed to it.

A central issue in the hearings will be the role of dioxin, and unavoidable contaminant of 2, 4, 5-T and Silvex, in producing the health hazards of the pesticide. Dow's pretrial press packet cites the respected British medical journal Lancet as noting: "For the amount of [dioxin] contaminating the 2, 4, 5-T used in our environment there is not evidence of harm." EPA's proposed ban, Lancet said, is "a waste of effort, resources and credibility to cry wolf . . . when there is no wolf."

Dioxin in much higher concentrations was found in Agent Orange, a Dow herbicide used in the Vietnam war. The company faces 31 lawsuits filed by families of veterans who claim they have suffered health damage from exposure to Agent Orange. Blair said, however, that the cases are unrelated to the 2, 4, 5-T hearings.

The company does not argue that 2, 4, 5-T is risk free. "No chemical, including 2, 4, 5-T, is without risk," said Dow Attorney Edward Warren in a statement, "yet if every substance shown to pose any risk were banned, our economy -- indeed, even our society -- would be brought to its knees."

It is Judge Finch's job to decide how much risk is too much.