Google said it was prepared to make changes to its AdSense service that mimicked Quigo's approach, an unusual step for a company accustomed to mapping the terrain in every aspect of its business. 

In the next few months, Google's advertiser reports will begin listing the sites where each ad runs, Ms. Malone said. She added that advertisers on the Google networks would soon be able to bid on contextual ads on particular Web sites rather than simply buying keywords that appeared across Google's entire network.