The Metro transit authority, in a move to improve Metrobus service for handicapped riders, decided yesterday for the first time to buy buses equipped with special lifts designed to help passengers in wheel chairs get aboard.

The new buses, Metro officials said, will have collapsible front steps that can be instantaneously converted into hydraulically or electrically operated lifts. Handicapped riders would maneuver their wheelchair onto the lift, which would raise them aboard the bus or lower them to the ground after a bus ride.

The first Metrobuses outfitted with lifts would comprise only a small part of the area's bus fleet with the transit agency initially planning to purchase as few as 120 of the specially designed buses. Metro now operates a 1,800-bus fleet. Eventually, all Metrobuses may be specially equipped for handicapped riders, officials say.

Lift-equipped buses are only starting to gain acceptance among major transit systems in the United States. Metro officials say they intend to move cautiously in buying the largely unproven equipment. Their main concern, they say, is that they newly designed lifts may be susceptible to troublesome breakdowns or malfunctions.

There are increasing signs, however, of federal emphasis on making big-city transit systems easier for handicapped riders to use. Secretary of Transportation Brock Adams recently announced a new policy under which all public buses purchased with federal funds after September, 1979, must be designed for ready access by handicapped and elderly passengers.

Metro's board of directors voted yesterday to buy at least 120 lift-equipped buses to replace some of its oldest and most obsolete vehicles. A larger purchase is possible, however, because the board decided it might buy an additional 108 such buses if the devices appear to be working satisfactorily in other transit systems that use them.

In addition, the board agreed to purchase 20 smaller buses, also outfitted with lifts, for use on the mid-city Downtowner route, the M-8.

Richard Heddinger, one of several handicapped persons who attended the board session, applauded the board's decision as a significant first step, but complained that "the action doesn't go far enough." He contended that the board should have moved immediately to buy 223 lift-equipped buses, instead of deferring a decision on the second group of 108.

Heddinger is a local board member of the National Paraplegic Foundation and a plaintiff in a suit that forced Metro to install elevators for handicapped passengers in the new subway system.

The lift-equipped buses are expected to be in operation here by next summer. Metro officials estimate their cost at about $20 million, an amount expected to be largely subsidized by the federal government. Both Metro and federal transportation officials say the U.S. subsidy for the bus purchase is virtually assured.

The St. Louis bus system this month became the first in the United States to put a substantial number of life-equipped buses into regular service, although such buses have been used in small, specialized projects elsewhere for some time.

A spokeswoman for the Urban Mass Transportation Administration said substantial members of lift-equipped buses also have been ordered by transit agencies in Los Angeles. Detroit, Altanta, Seattle, Stamford, Conn, New Haven and Santa Clara, Calif.

In St. Louis, where 60 buses with lifts were put into service Aug. 15, few passengers have used them - only about 15 so far - and some mechanical troubles have occurred, according to a spokeswoman for the Bi-State Development Agency, which runs the bus system.

The malfunctions however, have taken place only during training exercises and during nightly maintenance checkups. Shirley Browne, the agency's public information director, said.