There were a few testy moments. Mike Lee, Republican of Utah, showed a chart with the rankings for Google Product Search in hundreds of shopping searches, compared with the rankings of three comparison shopping sites, Nextag, Pricegrabber and Shopper. The rivals’ rankings varied widely, while Google’s service was consistently ranked third.

Mr. Schmidt first replied that the chart was an “apples to oranges” analogy, because the Google service steers users to specific products and is not a shopping comparison site.

Unconvinced, Mr. Lee said, “You cooked it so you are always No. 3.”

Mr. Schmidt replied, his voice tightening, “Senator, I can assure we haven’t cooked anything.”

Google’s competitors testified in a second panel, after Mr. Schmidt, an arrangement that Google requested and the subcommittee accepted. The competitors described a different world than Mr. Schmidt portrayed, saying Google has immense market power and uses it.

Jeffrey Katz, the chief executive of Nextag, said that Google was “an outstanding partner to us for many years,” but that the relationship has become strained as the search company expanded. Google’s business interests, he said, conflict with its engineering commitment to an open-for-all Internet.

“But what Google engineering giveth, Google marketing taketh away,” Mr. Katz said. “Today, Google doesn’t play fair. Google rigs its results, biasing in favor of Google Shopping and against competitors like us.”

The issue, he said in a separate interview, is subtle and does not affect all Google searches, mainly ones related to buying goods or services. “When you search for ‘running shoes’ or ‘digital camera,’ Google transforms itself from an independent search engine to a commerce site,” Mr. Katz said. “But that is not what happens when you type in a search for, say, ‘kidney dialysis.’” Jeremy Stoppelman, the chief of Yelp, said sites like his have to cooperate with Google because it is the gateway to so many users. About half of Yelp’s visitors come through Google search.