Robert F. Kennedy Jr., testifying before Congress for the first time as health secretary, also said he did not think Americans should be taking “medical advice from me.”

Health Secretary Robert F. Kennedy Jr. delivered a defiant defense on Wednesday of his drastic overhaul of federal health agencies, insisting to members of Congress that he had “not fired any working scientists” and was “not withholding money for lifesaving research” despite evidence to the contrary.

In back-to-back appearances before House and Senate committees, Mr. Kennedy, a longtime critic of vaccination, also made clear that he did not think the health secretary should be in the business of making vaccine recommendations. He ducked questions about whether, if he had young children today, they would be inoculated against measles, chickenpox or polio.

“I don’t think people should be taking advice, medical advice from me,” the health secretary said.

After weeks of controversy about his plans for autism research, Mr. Kennedy also testified that federally funded studies should focus solely on identifying “environmental toxins” — a term that Mr. Kennedy’s critics say is code for vaccines.

“I’m told that there was a 20-to-1 research ratio for genetic causes over the past 20 years,” he said, adding: “I believe that was because they did not want to look at the environmental exposure because they were scared. So I don’t think we should be funding that genetic work anymore.”

Mr. Kennedy had come to Capitol Hill, his first appearance there since becoming health secretary, to promote President Trump’s budget for the next fiscal year. But his testimony devolved into a series of fiery exchanges with Democrats, who wanted to talk about the mass layoffs and cuts to research funding he has already imposed.

Engineered in part by Elon Musk and his team at the Department of Government Efficiency, the remake of the health department includes cutting 20,000 jobs — one-quarter of the health work force. It also collapses entire agencies, including those devoted to mental health and addiction treatment, and emergency preparedness, into a new, ill-defined “Administration for a Healthy America.”