Yahoo today announced that it will conduct a test of Google’s AdSense for Search service. Yahoo officials said that the tests will be done in the U.S. and allow the company to further investigate possible usage of Google’s AdSense system. Yahoo said that it expects the test to last up to two weeks using roughly 3-percent of Yahoo search queries.

The tests that are being conducted will run primarily on Yahoo’s main domain, yahoo.com. Tests will not include any of Yahoo’s other services or affiliates, nor will it include premium publishing partners.

If successful, search results on Yahoo’s own site will show text ads run by Google’s ad system. This is the biggest Google announcement by Yahoo since it stopped outsourcing search to Google several years ago.

The announcement comes during a time of trifle between Yahoo and Microsoft. The two companies have been back and forth with each other regarding Microsoft’s proposal to acquire the Internet search pioneer. Earlier today, major Yahoo shareholder Legg Mason publicly stated that he would back up Yahoo if Microsoft failed to offer more money. Yahoo’s board of directors rejected Microsoft’s bid twice now, leaving Microsoft little alternative. Jerry Yang said earlier this week that he would not sell his company to Microsoft or any other company, if an offer wasn’t up Yahoo’s expectations.

According to Yahoo:

Yahoo, a leading global Internet company, announced today that it will begin a limited test of Google Inc.’s AdSense for Search service, which will deliver relevant Google ads alongside Yahoo!’s own search results. The test will apply only to traffic from yahoo.com in the U.S. and will not include Yahoo!’s extended network of affiliate or premium publisher partners.

Yahoo indicated that users should not take this announcement as a definitive indication of any kind of partnership.