WASHINGTON — Saying it was “hush-hush” and “on the QT,” I. Lewis Libby Jr. disclosed the identity of a CIA officer to the then White House press secretary, Ari Fleischer, over lunch in July 2003, Mr. Fleischer testified yesterday.

The former chief of staff to Vice President Cheney who is on trial for perjury, Mr. Libby told a grand jury that he first heard the name of the CIA officer, Valerie Plame, from a reporter three days after the alleged conversation with Mr. Fleischer. Mr. Libby is not charged in the outing of Ms. Plame but with lying under oath about his discussions with reporters.

Mr. Fleischer told the jury that he and Mr. Libby met for lunch in the White House mess on July 7, 2003, the day after Ms. Plame’s husband, Joseph Wilson IV, published an opinion piece critical of the Bush administration’s justification in going to war with Iraq. In the article printed in the New York Times, Mr. Wilson said the CIA had dispatched him to Niger to investigate claims of concern to the vice president’s office that Iraq had purchased yellowcake uranium. The claims, which Mr. Wilson disputed, had formed the basis of the 16 words in President Bush’s 2003 State of the Union address that the White House was forced to retract that same week.

The two senior aides had never met for lunch, but Mr. Libby scheduled it to discuss Mr. Fleischer’s plans after his departure from the White House, which was weeks away. After talking “a little football,” Mr. Fleischer said Mr. Libby told him that it was Mr. Wilson’s wife — and not the vice president — who sent him to Niger. “I just remember him saying that she worked for the CIA and for the counterproliferation division,” the former spokesman testified.

“‘This is hush-hush. The is on the QT,'” he said Mr. Libby added. “‘Not many people know about this.'”

Mr. Fleischer said he thought Mr. Libby had uttered Ms. Plame’s name during the conversation, but he conceded under cross-examination by the defense that he was not sure. “Absolute certainty? No,” he told the court.

While the prosecution is accusing Mr. Libby of lying, the defense contends that he was too busy to remember the details of brief conversations that he had with reporters.

Mr. Fleischer was testifying under the cover of immunity from prosecution that he demanded and won after refusing to answer questions before a grand jury in January 2004.

The three hours of testimony marked a return to the spotlight that Mr. Fleischer had occupied for two-and-a-half years as President Bush’s chief spokesman. His experience was not lost on the defense attorney, William Jeffress Jr., who noted at the outset of his cross-examination that Mr. Fleischer had “a lot of practice” taking questions that were “sometimes hostile.”

Wearing a dark blue suit but no glasses, Mr. Fleischer appeared confident yet careful in his testimony. Often looking directly at the jury, he readily explained the workings of the White House press operation. Peppering some of his responses with “as I said earlier” or “as I have said before,” he resisted Mr. Jeffress’s efforts to expose contradictions between his testimony yesterday and his statements to the grand jury.

Mr. Libby did not specify that the CIA status of Mr. Wilson’s wife was classified or covert, Mr. Fleischer testified. He said he thought the vice presidential aide was telling him something “kind of newsy” that suggested there was “nepotism” at work in the CIA. “I never in my wildest dreams would have thought this information was classified,” he testified later.

Moreover, there was standard White House protocol for communicating classified information, which Mr. Libby did not follow, Mr. Fleischer said. “The people I worked with would always say, ‘This is classified. You cannot use it,'” he said.

After hearing a second mention of Mr. Wilson’s wife from Dan Bartlett, the White House communications director, Mr. Fleischer said he decided on his own to repeat the disclosure to an NBC News correspondent, David Gregory, and a Time magazine reporter, John Dickerson, while standing on the side of a road in Uganda during a presidential trip.

The reporters didn’t seem interested in the news, Mr. Fleischer said. “Like a lot of the things I said to the press, it had no impact,” he said, drawing laughter from a courtroom that included more than a dozen journalists.

Ms. Plame’s identity first appeared in a column by Robert Novak on July 14, 2003, but Mr. Fleischer said he didn’t become concerned until he read about a federal investigation into the leak in late September. “I was absolutely horrified,” he said. “I thought, ‘Oh my God, did I play a role in outing a CIA officer?'”

The government began questioning of Mr. Cheney’s current chief of staff, David Addington, late yesterday afternoon. His testimony is expected to conclude today and be followed by that of a former New York Times reporter, Judith Miller, who served 85 days in jail for refusing to disclose her sources in relation to the CIA leak.