Under pressure from Southern politicians and rejecting warnings by its staff scientists, the Environmental Protection Agency has reapproved the "emergency" use of a potent new pesticide against fire ants in Mississippi.

The decision could clear the way for the use of the pesticide ferriamicide in eight other Southern states infested by fire ants. EPA rulings on those states' applications are some weeks away, an agency spokesman said.

The Mississippi ruling and the events leading up to it illustrate the continuing clash between politics and science -- with politics, in this case, the winner.

Ferriamicide, developed by Mississippi's state-owned chemical plant, is made with Mirex, a product banned in 1976 by the EPA because it is thought to cause cancer and birth defects. Mirex, however, is widely viewed as the most effective tool against fire ants.

New Canadian research data -- not considered by the EPA before its Mississippi approval -- indicates that as ferriamicide breaks down it produces a compound 10 to 100 times more toxic than Mirex.

The Canadian data became the basis for an Environmental Defense Fund request this week that the EPA rescind its Mississippi decision. An EPA spokesman said it had not decided on a response as of last night.

"This product is completely untested -- the decision makes a travesty of the regulatory process," said William A. Butler, general counsel for the public-advocacy law firm.

The EPA approved emergency use of ferriamicide in Mississippi twice last year but the pesticide never was applied because of a suit brought by the EDF. A federal judge in September held that the EPA had wrongfully accepted comment after the comment period had closed and ordered a reprocessing of the Mississippi request by the EPA.

The EPA's decision to allow once-only application of the pesticide this spring in Mississippi, a major point of fire ant infestation, became final with a Jan. 30 ruling by Barbara Blum, deputy administrator. She used an authority that allows "emergency" applications of unapproved pesticides.

Although Blum put strict, but hard-to-enforce, limits on handling and application of the chemical in Mississippi, she conceded that ferriamicide presented risks to human health.

But she also held that the risks were outweighed by the benefits of blitzing the pestiferous fire ants, which bite and sometimes infect humans and livestock and whose mounds impede farming.

Blum's decision came after months of intense lobbying by members of Congress and officials in eight other Southern states who want ferriamicide approved for use against fire ants in their areas.

The EPA has insisted for months that its earlier rulings for ferriamicide -- delayed by EDF litigation -- were based solely on the merits of the fire ant problem and had nothing to do with politics.

But the record of letters, memos, telephone calls and scientific data turned over to the U.S. District Court here last year comprises a stunning documentary of executive branch reaction to pressure from Capitol Hill.

Over the past two decades, the South's fight against the ever-spreading fire ants has become as important politically, as, say, the issue of crime in the big cities or automobile air pollution in Detroit.

A semi-panic was set off in Washington and across the South when Mississippi agreed, because of the health perils, to end the production of Mirex by the middle of 1978.

The challenge was to find another powerful agent that could wipe out the fire ant, which has moved steadily across the South since it slipped into Alabama from Latin America 50 or more years ago.

Mississippi's solution was ferriamicide, which it developed in the state-owned plant that it bought for $1 when Allied Chemical Co. decided it no longer wanted to manufacture Mirex.

Led by Jim Buck Ross, the state agriculture commissioner, and Reps. G. V. (Sonny) Montgomery (D-Miss.) and Billy Lee Evans (D-Ga.), Southern politicians began bombarding the EPA for a favorable ruling on ferriamicide.

The EPA files show that at least 14 senators, 47 representatives, three governors and the agriculture commissioners from the nine fire ant infested states pushed for approval.

Some 20,000 letters from Mississippians, inspired by Ross' appeals, flooded the EPA with similar requests. The agency -- and Blum in particular -- quickly got the message, despite warnings from scientists at the EPA about the pesticide.

EPA's chemistry branch said testing was inadequate and that residues of Mirex could find their way into the food chain. It opposed the emergency approval.

Similar reports came from other offices. The toxicology branch said it could not estimate human hazards because of insufficient data. The fish and wildlife section said ferriamicide use on 17 million acres -- as proposed by the nine states -- was "totally unjustified."

Dr. D. C. Villeneuve's Canadian data, based on laboratory tests using rats, raised other disturbing questions -- the possibility that photomirex, a compound that occurs as ferriamicide decomposes, could cause cancer, affect male reproductive capability and cause liver damage. He described the compound as 10 to 100 times more toxic than Mirex, the basic ingredient of ferriamicide, and five times more toxic than Kepone, another Mirex derivative.

But while Mississippi was phasing out Mirex, the pressure was building at the EPA. Evans proposed a bill to allow Mirex usage, but Blum and Douglas Costle, head of the EPA, persuaded him to back off by promising early action on an alternative pesticide.

The message had been received loud and clear long before that, however. A year ago, Blum wrote a memo to Steven D. Jellinek, an assistant administrator, saying, "Legally, we probably can't approve it [ferriamicide]," But she noted a "political problem" with crucial Southern legislators holding sway over other pesticide-control amendments the EPA wanted passed.