After years of litigation and lobbying, the nation's largest broadcasters, cable companies and other media outlets have begun winning important changes to federal rules that restrict their ability to grow larger and to dominate new markets.

The changes achieved by the powerful media and telecommunications companies reflect a new regulatory climate in Washington, both at the Federal Communications Commission and before a federal appeals court here.

In a marked departure from decades of Supreme Court opinions on the subject, the agency and the appeals court have become significantly more sympathetic to the free- speech rights of corporations and more skeptical of the role of government in promoting diversity in mass media....

The next industry victory is expected this week, when the communications agency is scheduled to relax a rule that for decades has prohibited one television network from buying another.

And within a few weeks, officials said, the agency will begin to loosen a 26-year-old regulation restricting a company from owning a television station and a newspaper in the same market.

In recent weeks, the federal court, the United States Court of Appeals for the District of Columbia, has handed two big victories to the largest companies in the cable and broadcasting industries. It struck down the rules that limit how big a cable company can grow, and it expressed grave doubts about comparable rules that have limited the nationwide size of broadcasters.