Food and Drug Administration officials acknowledged yesterday that they are unable to cope with the widespread abuse of animal drugs.

In testimony before a House Government Operations subcommittee, FDA officials did not quarrel with estimates that there are 20,000 or more brands of animal drugs being used, while only 2,500 have been approved by the agency. They recognized the possibility that the illegal drugs might pose a threat to the nation's meat, milk and egg supply.

"We're not able to monitor for all the [drug] products that might be used," said Dr. Lester M. Crawford, head of the FDA's Center for Veterinary Medicine. The law demands such monitoring, but Crawford told Rep. Ted Weiss (D-N.Y.) that the problem "is not completely under control and never has been. We do the best we can."

"Can the FDA say with any degree of accuracy which of these drugs are going into food-producing animals?" Weiss asked.

"To give you a precise figure at this point -- I could not," Crawford responded.

The testimony came on the first day of a two-day hearing by Weiss' intergovernmental affairs subcommittee, capping a lengthy investigation into the effectiveness of animal drug safety laws.

It is clear that chemicals do pass up the food chain to humans and sometimes cause illness.

An example occurred this month when more than 200 people fell ill after eating California watermelon contaminated with the pesticide aldicarb.

The effect of animal drugs in meat products is hard to measure, however. Most are linked to diseases that develop over long periods of time, such as cancer, and so are difficult to track to a single source.

The connection between animal use and human disease has not been well established.

Crawford said the FDA's job is complicated by the fact that the agency does not know what drugs are on the market.

While he did not disagree with the panel's estimate of 20,000 animal drug products in use, he said the FDA has no comprehensive list of such products even though the law requires manufacturers to list them with the agency.

"Isn't the failure to list a drug a prosecutable offense?" Weiss asked.

"Yes," Crawford replied. He said the agency is working on a central listing and may have some results by next year.

"Putting it in its proper priority, we didn't get around to it," he said. "We've been very slow and it's because of prioritization."

Committee investigators said they also found that:

*Perhaps 3,500 of the unapproved drugs may leave residues in meat, milk and eggs capable of causing cancer or birth defects in humans who eat the foods.

*Many of the products are being sold over the counter by veterinarians or their employes, without regard to their intended use and often in violation of laws requiring prescriptions for their purchase.

*Undercover agents were able to buy a prescription-only drug over the counter in 33 of 37 veterinary clinics visited in eastern Iowa, without talking to a veterinarian or disclosing what the drug was to be used for, said Clifford G. Shane, a regional enforcement official for the FDA.

*In several cases clinics sold oxytocin, a powerful labor-inducing drug, to a female agent in maternity clothes -- without warning her that the drug can induce spontaneous abortion if handled by pregnant women, Shane said.

A debate over the abuse of animal drugs has been seething for years because of the widespread and profitable adoption of livestock practices that rely on heavy doses of drugs.

The drugs speed weight gain and prevent disease outbreaks among animals raised in close quarters.

According to FDA officials, the problem has intensified in recent years as financially pressed farmers increasingly elect to treat their own animals and veterinarians expand into the retail drug business.

Crawford defended his agency's enforcement efforts and deflected Weiss' suggestion that the agency is understaffed.

"We think we've made steady progress," he said. "We've certainly made a steady effort."

But he acknowledged that the federal government does not regularly check for residues of several drugs that are suspected of being cancer-causing agents.

Among them are dimetridazole and ipronidazole, drugs that the FDA officials conceded are being used by hog producers even though they are approved for use only in turkeys.

According to documents released by the subcommittee, the agency recently declined, "in the absence of illegal tissue residues or other compelling circumstances," to take action against a Missouri hog producer who illegally used dimetridazole.

"Don't you think the unapproved use of a carcinogen represents compelling circumstances?" Weiss asked, noting that the FDA has known of the product's cancer-causing properties for as long as seven years and that agency officials have repeatedly urged that it be banned.

"It's a long process to get drugs off the market once they're on," Crawford said. "It's a much easier process to approve them."