he rush to develop new vaccines and drugs to counter bioterrorism will pose another challenge: How should such drugs be tested so they can be approved for use?

Normally, experimental drugs for diseases like cancer or arthritis are tested in clinical trials on people with the disease to see if the drugs are safe and effective. But there is no large group of people suffering from diseases like smallpox and anthrax, which are caused by pathogens that might be used by bioterrorists. And deliberately exposing people to lethal agents would be unethical.

"You don't have a situation where you can simply test the drug in a population that has the problem," said Dr. Jane E. Henney, who was commissioner of the Food and Drug Administration until January.

The issue poses ethical, legal and scientific questions, experts said. Given the current public alarm about anthrax and smallpox, there is likely to be pressure on the F.D.A. to relax its testing requirements or to allow unapproved drugs to be used. But the testing requirements do screen out many dangerous and ineffective medicines. Experimental drugs to protect against biological and chemical attacks were given to American soldiers during the Persian Gulf war, and there is some evidence suggesting that soldiers may have suffered side effects.

The F.D.A. is now rushing to put into effect a rule first proposed in late 1999, during Dr. Henney's tenure, that would allow data from animal studies to be used in place of human testing when it is impossible or unethical to do tests on patients.

But the agency is reluctant to relax its standards too much. Even though the nation is now in the midst of an anthrax attack, the only manufacturer of anthrax vaccine, the Bioport Corporation of Lansing, Mich., has been unable to supply its product because its factory has failed F.D.A. safety inspections. Earlier this month, BioPort applied for another inspection, which could take place in coming weeks.

Dr. Janet Woodcock, director of the F.D.A.'s Center for Drug Evaluation and Research, said the agency had faced similar questions before. When patients with cancer and AIDS pushed for more access to experimental drugs, the F.D.A. adopted procedures to speed approval of drugs for life-threatening illnesses and allowed for some experimental drugs to be made available.

"There's a balance between having access to therapy and having confidence that there is at least some chance that it will help you," she said.

The new rule the F.D.A. will soon put into effect applies to drugs for biological, chemical and nuclear agents. Healthy human volunteers would still be used to test the safety of a drug since the drug itself is not likely to cause a fatal disease. But to prove a drug works, which would require exposure to the pathogen, animal tests can be used if there is reason to believe the results will be applicable to humans.

The rule could be tested soon. Several major drug companies said this week that they would sell or donate large quantities of medicines to the government to fight anthrax provided the F.D.A. approved the drugs for that use.

Dr. Arthur O. Anderson, the chief ethics officer for the Army's germ warfare defense program, said the Army was now going through 40- year-old data on vaccines it developed, so that applications can be made to the F.D.A. to get the vaccines approved. Dr. Anderson, who is chief of the office of human use and ethics at the Army Medical Research Institute of Infectious Diseases at Fort Detrick, Md., said the F.D.A.'s previous procedures made it impossible to win approval for a vaccine against botulism that the Defense Department wanted to give soldiers during the gulf war.

"A whole group of people would have to die in order to prove that this is efficacious," he said. "Totally immoral."

The F.D.A. has already approved a new use for one drug without requiring human clinical trials. Cipro, the antibiotic now in great demand as a treatment for anthrax, was approved for that use last year on the basis of standards called surrogate markers. These markers indicate that a drug is working before ultimate proof of its effectiveness. The Defense Department tested Cipro on monkeys exposed to anthrax. The government then measured whether people could get a concentration of the drug in their blood equivalent to what worked in the animals.

Dr. Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, said surrogate markers are also being used to test whether the existing stock of smallpox vaccine can be diluted and still be effective. Instead of being exposed to smallpox, volunteers in the studies receive the vaccine and then get checked to see if they get a characteristic scabbing of the skin that has been definitively associated with the vaccine's effectiveness, and to see if they make antibodies to smallpox.

Dr. Fauci said he thought a similar approach will be used to try to win approval for a new smallpox vaccine being developed by Acambis (news/quote), a British company, for the Department of Health and Human Services. Other companies' vaccines will also have to be tested. Further evidence may come from tests in monkeys.

Some ethicists say it would be permissible to use experimental drugs in certain circumstances. Dr. Jonathan D. Moreno, director of the center for biomedical ethics at the University of Virginia, said that in a severe emergency, it might even be ethical to expose fully informed volunteers to pathogens to test a new drug or vaccine.

Indeed, the Army's germ warfare program at Fort Detrick has long exposed volunteers to pathogens like Q fever and tularemia in a program called Project Whitecoat, which studied vaccines and protective equipment, and in subsequent tests. Dr. Anderson of the Army said exposure was confined to pathogens for which treatments were available.

John Wilson, a participant in Project Whitecoat, recalled testing a safety suit by breathing contaminated air in a huge stainless steel chamber. "The people who volunteered for this had a lot of respect for the country and a lot of nerve," said Mr. Wilson, a retired insurance company executive in Knoxville, Md. "Yes, we were scared, but we decided to pay the price for the country. And we knew we would be taken care of."