The US news magazine Time has agreed to hand over notes made by one of its reporters to an investigation into the unmasking of a covert CIA agent.
Both reporters made inquiries about the source of the leak
Time's editors made the decision after a US Supreme Court ruling left one of its reporters facing 120 days in jail.
Reporter Matthew Cooper, together with Judith Miller of the New York Times, probed how the agent's name was leaked.
The New York Times said Time's decision to hand over the notes was without precedent in living memory.
The name of the agent, Valerie Plame, was leaked in 2003 in a row over evidence used by the US government to justify the invasion of Iraq.
In a statement, Time's editor-in-chief, Norman Pearlstine, said the magazine was handing over notes and information on sources out of respect for the US constitution.
"The same constitution that protects the freedom of the press requires obedience to final decisions of the courts and respect for their rulings and judgments," he said.
"We are not above the law," Mr Pearlstine told CNN.
Time's decision should end any threat of jailing Cooper, Mr Pearlstine added.
The publisher of the New York Times, Arthur Sulzberger Jr, said he was disappointed with Time's decision.
He pointed out that in 1978 a New York Times reporter spent 40 days in jail and the newspaper was heavily fined for refusing to reveal sources.
"Our focus is now on our own reporter, Judith Miller, and in supporting her during this difficult time," Mr Sulzberger Jr said.
The leak of Ms Plame's name was not made to Cooper or Miller, but they came to the attention of a special prosecutor investigating the leak because of their inquiries.
They refused to co-operate with the investigation, claiming they should not have to reveal their sources because of press freedoms guaranteed in the US Constitution.
That defence was over-ruled by a court in Washington.
Ms Plame's work as a covert CIA operative working on the issue of weapons of mass destruction was revealed by the nationally-syndicated conservative newspaper columnist Robert Novak.
Eight days earlier her husband, former ambassador Joe Wilson, criticised President George W Bush's claim in his 2003 State of the Union address that Iraq sought to buy uranium from Niger.
Mr Wilson claimed his wife's name was leaked in retaliation for his article.
The leak, which was a federal offence, sparked an investigation into its source, which has involved Cooper and Miller.
There has been no indication that Novak, who first reported the leak, is facing any censure.
The New York Times says this indicated that he is likely to have co-operated with the investigation.