Some members of the panel called McNamee "a liar" and a "drug dealer." McNamee was also Pettitte's personal fitness coach and has said he injected Pettitte with human growth hormone.
"I view you as a police officer who was a drug dealer," Rep. Christopher Shays, a Connecticut Republican, told McNamee.
Shays: "Mr. McNamee, you are a drug dealer."
McNamee: "That's your opinion."
But McNamee added: "I have no reason to lie and every reason not to."
McNamee has provided federal law enforcement authorities with syringes and other medical evidence that his attorneys say is likely to link Clemens to steroids.
Richard Emery, a McNamee lawyer, said after the hearing that some members - he named Republicans Reps. Dan Burton of Indiana, Darrell Issa of California and Virginia Foxx of North Carolina - had seemed to be swayed by pre-hearing visits that Clemens had made to meet lawmakers and argue his case.
"God knows whose water they were carrying, but it's pretty clear the lobbying effort worked. Brian knows he told the truth here. He can stand tall," Emery said.
Elements of the day seemed surreal, as when Clemens emerged from the hearing and was handed a baseball to autograph by a fan waiting in the Rayburn House Office Building corridor. He signed it.
Foxx and other members wondered why Congress was devoting so much time to baseball. "This circus could only impress P.T. Barnum," said a statement by Rep. Patrick T. McHenry, a North Carolina Republican.
But Waxman said the committee held the hearing "to close the chapter on looking at baseball's past."
Waxman seemed skeptical of Clemens' repeated denials. He said Clemens had made conflicting statements about why he didn't cooperate with investigators for former Sen. George J. Mitchell, who released a report in December on steroids in baseball.
The committee also provided a doctor's statement that a soreness Clemens had once complained of on his buttocks could have been caused by steroids injections.
Said Waxman: "Someone isn't telling the truth. If Mr. McNamee is lying, then he has acted inexcusably and made Mr. Clemens an innocent victim. If Mr. Clemens isn't telling the truth, then he has acted shamefully and unconscionably smeared Mr. McNamee.
"I don't think there's anything in between."