A 15-month inquiry by a top House Democrat has found that enforcement of the nation's food and drug laws declined sharply during the first five years of the Bush administration. 

For instance, the investigation found, the number of warning letters that the Food and Drug Administration issued to drug companies, medical device makers and others dropped 54 percent, to 535 in 2005 from 1,154 in 2000. 

The seizure of mislabeled, defective or dangerous products dipped 44 percent, according to the inquiry, pursued by Representative Henry A. Waxman of California, the senior Democrat on the House Government Reform Committee.

The research found no evidence that such declines could be attributed to increased compliance with regulations. Investigators at the F.D.A. continued to uncover about the same number of problems at drug and device companies as before, Mr. Waxman's inquiry found, but top officials of the agency increasingly overruled the investigators' enforcement recommendations.

The biggest decline in enforcement actions was found at the agency's device center, where they decreased 65 percent in the five-year period despite a wave of problems with devices including implantable defibrillators and pacemakers.

"Americans have relied on F.D.A. to ensure the safety of their food and drugs for 100 years," Mr. Waxman said. "But under the Bush administration, enforcement efforts have plummeted and serious violations are ignored." 

David K. Elder, the director of the agency's Office of Enforcement, explained that the F.D.A. had increasingly focused on the most serious violations.

"As a result of F.D.A.'s focus on those firms and those violations that present the highest risk to consumers and public health," Mr. Elder said in a statement, "the agency has taken prompt, targeted and aggressive action against firms that are in violation of law."

Jack Calfee, a resident scholar at the conservative American Enterprise Institute, said the decline in the statistics was meaningless because most of the violations involved paperwork problems.

"I doubt that it makes a significant difference in the safety of drugs or other products," Mr. Calfee said.

Mr. Waxman began his inquiry after Congressional hearings in 2004 suggested that the agency was partly to blame for a shortage of flu vaccines. His staff requested thousands of documents from the F.D.A.

The investigation found that by almost every measure, enforcement actions had significantly declined from 2000 to 2005. The lone exception was in the number of products that had to be recalled from the market: that increased 44 percent.

"Since one of the goals of an enforcement system is to deter violations and keep dangerous products off of the market," the report said, "the increase in recalls is not a hallmark of effective enforcement."

In one prominent case, in December 2000, a worker at a nursing home in Xenia, Ohio, mistakenly hooked up a tank of nitrogen gas to the home's oxygen delivery system. Four residents died. 

In the months that followed, investigators for the agency concluded that the company that delivered the tanks, BOC Gases, was partly to blame for the mix-up, given what they deemed inadequacy of the company's own controls and employee training. Indeed, BOC had a "corporate-wide problem," F.D.A. documents at the time said. The investigators recommended prosecution, but the agency took no enforcement action.

Kristina Schurr, a spokeswoman for BOC, said that the company's controls had not been to blame but that in any case it had improved its procedures since then.

Several former top officials of the agency attributed the decline in enforcement actions to budget problems.

"This is a tragedy," said Peter Barton Hutt, a former general counsel of the F.D.A. who now represents drug companies and teaches food and drug law at Harvard. "Congress has failed to realize that our single most important government agency is being systematically dismantled."

Dr. Sidney M. Wolfe, director of the Health Research Group at the watchdog organization Public Citizen, noted that the agency now received about $380 million a year in fees from drug makers.

"The public," Dr. Wolfe said, "is getting the kind of F.D.A. that the industry is paying for them to get."