The talks represent a change of sides for Mr. Murdoch. Just days after Microsoft made its bid, he flew to the West Coast and had dinner with Jerry Yang, Yahoo’s chief executive, offering his assistance in fending off Microsoft.

A deal between them would have involved joining forces with Google to use its lucrative advertising system on the companies’ sites. Antitrust concerns have, however, all but scuttled those talks, at least in that form.

On Wednesday, Yahoo suggested that it might be willing to cede part of its core business to Google, an archrival, to remain independent.

Yahoo said it would begin outsourcing a small portion of its search advertising to Google. The limited test is meant to determine whether the company could extract more revenue if Google ran its search advertising system. The test results might also back Yahoo’s contention that Microsoft’s offer undervalues the company, a person briefed on the plan said.

In the two-week test, Yahoo will use Google’s search advertising system to deliver ads that appear alongside Yahoo’s search results. The test will involve searches conducted in the United States on Yahoo.com, not on any of the company’s search affiliates, and will be limited to no more than 3 percent of all search queries, Yahoo said in a statement. Yahoo also said there was no guarantee that the test would lead to a broader deal.

Microsoft immediately blasted the idea of a search advertising partnership between Yahoo and Google, saying it would be anticompetitive. “Any definitive agreement between Yahoo and Google would consolidate over 90 percent of the search advertising market in Google’s hands,” Microsoft said in a statement.

In Washington, Senator Herb Kohl, a Wisconsin Democrat who is chairman of the Judiciary Committee’s Subcommittee on Antitrust, Competition Policy and Consumer Rights, also warned about the potential anticompetitive implications.