Hundreds of buses, some of them new and others freshly renovated, will join the Metro system's aging fleet in the next months, raising hopes of fewer breakdowns and more dependable bus service.

"We've started to head in the right direction," said Theodore G. Weigle, Metro's assistant general manager for transit operations. The transit authority has embarked on a stepped-up effort to improve bus service, a plan that hinges partly on rejuvenating its fleet of 1,700 buses.

Some of Metro's buses are 22 years old and the fleet's average age is 10.8 years. "We want to drop that down to about 7 years," Weigle said.

The new buses beginning to arrive at Metro's sprawling garage in Northeast Washington represent the first step. Four new articulated buses--elongated vehicles that bend in the middle when turning corners--have been delivered by a West German-owned manufacturing company and 29 more are expected by the end of July.

At the same time, the first of 210 renovated buses will begin reaching Washington from a Chicago plant. These are older vehicles being rebuilt to last another eight years. Their engines are being overhauled, new starters and other equipment installed, and the buses will be freshly painted. They are due at a rate of 20 buses a month, starting in mid-July.

An additional 75 new buses, purchased by Metro from another West German-affiliated company, are scheduled for delivery in November and December. And Metro's maintenance crews plan shortly to begin rehabilitating 200 other buses that are part of its present fleet.

All of this is costing $40 million, largely financed by federal grants. But Washington-area officials say that these moves are essential to improving the transit system, especially in the District of Columbia where bus service is regarded as most unreliable. The new and rehabilitated buses will keep Metro's fleet at about its present number or slightly less, as older buses are retired from service.

Metro must fix up its "raggedy buses," D.C. Councilman Jerry A. Moore Jr., a member of the transit authority's board of directors, has asserted at recent board meetings. "They look like they just returned from a battlefield."

Added Hilda H. M. Mason, another Council member who also serves on Metro's board: "I do not use the buses unless I really have to. I take a taxi--and I'm a board member. I think it's a disgrace." Mason said that she felt sorry for D.C. residents who must ride the bus.

Such complaints are underscored by an array of bleak statistics, including recent figures dealing with bus breakdowns in Southeast Washington. Metro officials said the statistics indicate that, on average, each bus serving the Southeast area breaks down every four days.

Although Metro's recent bus purchases appear to have brightened prospects for the beleaguered transit system, they have sparked controversy. The transit authority's purchase of 75 buses from the West German-affiliated firm, Neoplan U.S.A. Corp., has been criticized by Joseph Alexander, a longtime member of Metro's board of directors, who is a Fairfax County supervisor and chairman of the American Public Transit Association.

Alexander contended that the deal was a mistake, citing evidence of major shortcomings in buses bought by other transit systems from the same firm. A company representative later attributed the problems to "growing pains" encountered in setting up plants in the United States, and said that the buses sold to Metro would be of high quality.

Articulated buses have previously caused maintenance problems for Metro. Four years ago, the transit authority bought 43 articulated buses from M.A.N. Truck and Bus Corp., the same company that is now delivering another 33 vehicles.

Air conditioning systems frequently malfunctioned in the older articulated buses, according to Metro officials. The buses' engines overheated and, transit officials said, the buses sometimes ran out of gasoline unexpectedly in the middle of their routes. Such mishaps, however, apparently no longer occur.

"We don't have any notable problems with them," a Metro spokesman said.

Cost was a key factor in the transit authority's decision to order renovated buses from the Chicago-based Blitz Corp. Metro will spend about $80,000 for each of these buses, compared with a typical price of $150,000 for a new bus. Metro's new articulated buses, which are 1 1/2 times as long as ordinary buses, cost $271,877 each.

Metro officials said they believe the rehabilitated buses will be more reliable than many newer models. The buses being renovated at the Chicago plant were made by General Motors and Metro officials said they have been very dependable.