Several members of Congress said yesterday that the National Cancer Institute has failed to ensure that all cancer patients are fully informed of the risks involved in treatment with highly toxic experimental drugs, but federal officials maintained that such incidents are not widespread.

Rep. Henry A. Waxman (D-Calif.) told a House hearing that some cancer patients do not understand "that they are being used as a guinea pig to see how much toxic effect a drug will have and that any value to them is remote." And Rep. Albert Gore Jr. (D-Tenn.) criticized the cancer institute for waiting nine months to report that one experimental drug caused serious kidney damage in some human subjects, even while the same drug was being given to other patients.

Vincent T. DeVita, the cancer institute's director, acknowledged under questioning that the institute has made some mistakes in the monitoring of experimental drugs. But he stressed that fewer than 3 percent of the 1,400 cancer patients who participated in preliminary chemotherapy tests over the last 18 months have died from the experimental drugs.

However, he said, only 9.5 percent were significantly helped by these drugs.

The joint hearing of the House Commerce subcommittee on health, chaired by Waxman, and the House Science and Technology investigations subcommittee, headed by Gore, was called to explore a variety of medical and ethical questions raised in a recent series in The Washington Post about the dangers of experimental anticancer drugs.

The congressmen heard new evidence that cancer patients at M.D. Anderson Hospital in Houston were given a highly toxic experimental drug and misled about the side effects. Alexander M. Capron, director of The President's Commission for the Study of Ethical Problems in Medicine, said the hospital improperly tested this drug, known as MTHHF, on six human subjects, even though the NCI had approved it for use only in animals.

Capron said the patients were given a consent form that assured them that the drug "will be free of toxic effects." In fact, he said, earlier studies on dogs and monkeys had found that MTHHF caused diarrhea, dehydration, weight loss, incontinence, loss of appetite and rectal bleeding.

"How in the world can that be translated as 'free of toxic effects?' " Gore asked.

Capron said the cancer institute didn't discover the human testing until the researcher published a paper on it last June. He said the institute later suspended the study but took no further action against the Houston hospital.

DeVita insisted that preliminary, or Phase I, studies do have a therapeutic effect and have produced some successful drugs that previously were regarded as hopelessly toxic. He challenged as inaccurate the Post report that experimental drugs have been implicated in the deaths of 620 cancer patients, saying it is often difficult to distinguish the effects of the drug from the effects of the cancer itself.

Asserting that 46,000 cancer patients were saved last year by treatment that included drugs, DeVita said that new drugs "are tested first in people who desperately need help. Fully 90 percent of patients in Phase I trials who fail to respond to a drug die within a year."

Gore sharply criticized the cancer institute for waiting nearly a year to tell the Food and Drug Administration that another experimental drug, called MeCCNU, had caused severe kidney damage in 20 children who received it.

"We were wrong on that issue," DeVita said. "We were tardy in not telling the FDA." One FDA scientist, Robert S. Young, said the cancer institute has "fostered a research environment in which the rights of human subjects are not fully respected." He said some cancer patients don't realize they are being given drugs that their own doctors would refuse to take if they had cancer.

Dr. Emil J. Freireich of M.D. Anderson Hospital defended his facility for conducting the unauthorized human tests with MTHHF. "There was never any intent to deceive," he said. "We corrected that abuse and will prevent it from ever happening again."

But when Freireich later suggested it would be unethical to give cancer patients a drug with no therapeutic effect, Gore shot back: "Yet here are six patients where you've done just that."

Freireich blamed what he said was oppressive federal regulation for discouraging scientific progress, saying that "These regulations are in fact harming the very patients they are designed to protect." He also said that patient consent forms were scaring cancer victims by revealing "the intimidating details" of their treatment.

Some witnesses said the chemotherapy controversy would deter cancer victims from seeking treatment. Dr. John E. Ultmann, a professor of medicine at the University of Chicago, called the Post series "a very inaccurate, unbalanced recitation of partially verified stories, taken out of context . . . that do no more than scare everyone."

Dr. Gail J. Povar, a clinical internist and teacher of medical ethics at George Washington University, said the question of informed consent is troublesome because terminal patients don't always heed their doctors' warnings. "Is a person whose only real alternative is death 'free' to decide?" she asked. "A piece of paper in no way constitutes informed consent. There is no consent form that can guarantee that the patient hears, reads, processes and understands that information."