The Food and Drug Administration does very little to ensure the safety of the millions of people who participate in clinical trials, a federal investigator has found. The inspector general of the Department of Health and Human Services, Daniel R. Levinson, said federal health officials did not know how many clinical trials were being conducted, audited fewer than 1 percent of the testing sites and, on the rare occasions when inspectors did appear, generally showed up long after the tests had been completed. The F.D.A. has 200 inspectors, some of whom audit clinical trials part time, to police an estimated 350,000 testing sites. Even when those inspectors found serious problems in human trials, top drug officials in Washington downgraded their findings 68 percent of the time, the report found. Among the remaining cases, the agency almost never followed up with inspections to determine whether the corrective actions that the agency demanded had occurred. In many ways, rats and mice get greater protection as research subjects in the United States than do humans," said Arthur L. Caplan, chairman of the department of medical ethics at the University of Pennsylvania. Animal research centers have to register with the federal government, keep track of subject numbers, have unannounced spot inspections and address problems speedily or risk closing, none of which is true in human research, Mr. Caplan said. Because no one collects the data systematically, there is no way to tell how safe the nation's clinical research is or ever has been. The drug agency oversees just the safety of trials by companies seeking approval to sell drugs or devices. Using an entirely different set of rules, the Office for Human Research Protections oversees trials financed by the federal government. Privately financed noncommercial trials have no federal oversight.