ASHINGTON, Jan. 14 — Michael K. Powell, the chairman of the Federal Communications Commission, reassured Congress today that there would not be radical changes in rules governing local phone service, high-speed Internet access and ownership of media outlets, but Democratic and Republican senators said they were worried that anticipated changes could hurt consumers.

The commission's efforts to foster the growth of digital communication will be "guided exclusively by the public interest, and resist the pressure to view our exercise as awarding benefits" to corporate interests, Mr. Powell told a packed Senate Commerce Committee hearing.

He derided as "melodramatic" reports of wholesale rule changes, saying such speculation is not necessarily "a reflection of what the majority of the commission thinks."

All five commission members made a rare appearance at the hearing to face questioning about how they intend to alter the communications landscape. The combination of technological changes and company failures, including the collapse of WorldCom, as well as judicial rejection of some commission rules have added to the importance of coming federal rule changes.

It was clear that any changes will be hotly disputed. Senator Ernest F. Hollings, the South Carolina Democrat who is chairman of the panel until the Republican changeover is completed, criticized the F.C.C. for "considering radically revising the rules of the game."

Senator John McCain, Republican of Arizona, who is to take over as committee chairman, said the agency would make "monumental decisions" this year that "will shape the future of communications forever."

A crucial area being reviewed is the federal limit on concentrated ownership of television stations and newspapers. "If you get it wrong, we will have much less competition and much more concentration," warned Senator Byron L. Dorgan, Democrat of North Dakota. "And the American consumer will suffer grievous injury."

Mr. Powell and other commissioners said they were committed to diversity, but Mr. Dorgan and other senators warned against concentration of media outlets.

"When you talk about more voices, are you talking about more voices by one ventriloquist?" Mr. Dorgan asked the commissioners.

Mr. Powell said the commission would not eliminate all media ownership rules, which restrict a newspaper from owning a television station in the same city or a media conglomerate from owning two television networks. Other rules specify that a broadcaster may not own television stations that broadcast to more than 35 percent of the nation's homes, and a broadcaster may not own two television stations in the same market unless there are at least eight other competitors.

Mr. Powell noted that the courts had already struck down some of the rules. The commission must now justify those rules, he added, or they will be set aside.

He acknowledged that he was concerned about growing concentration in radio station ownership, where a few large companies bought up many local stations after Congress relaxed the rules in the Telecommunications Act of 1996.

"I am troubled by the concentration in radio," he responded to Senator Ron Wyden, Democrat of Oregon, who said he worried about the trend being duplicated in the television market. Mr. Powell, a former antitrust lawyer, said that the way the agency calculated ownership levels in local markets could be adjusted, and noted that media company mergers and acquisitions are still subject to approval by regulators.

"The Senate won't support a monopoly or duopoly," said H. Russell Frisby Jr., president of CompTel, a trade association that represents AT&T, WorldCom and Sprint, after the hearing.

Some of the most pointed comments came early in the hearing when Mr. Hollings criticized Bell companies for trying to change F.C.C. rules that require them to lease elements of their networks to competing companies. Bell companies like Verizon and BellSouth estimate they lose about $600 million a year by giving discounts to rivals.

"The Baby Bells are saying they are forced to rent their networks at below their costs, and they want to reverse" the deal that was made when Congress passed the telecommunications law in 1996, he said.

"Are you on the side of less competition and higher prices?" he testily asked Mr. Powell, who responded that "nothing we do will deny competitors access to the local network."