Ridership on Metro's buses and subways will drop in the year ending June 30 for the first time since 1973, when Metro began operating mass transit, according to Metro officials. Most of the unexpected decline is occurring on the new subway system, not the old bus system, the officials said.

Although the decrease is expected to be slight -- 2.4 percent for combined bus and subway ridership -- it threatens Metro's budget assumptions for two fiscal years and is certain to raise questions about the value of continuing to plow massive amounts of taxpayers' dollars into public transit.

The official ridership report containing the bad news will go to the Metro board next week, but the specifics of the decrease and the concerns about it were confirmed yesterday.

"We are concerned about it, and I don't know how to fully explain it," said Metro General Manager Richard St. Page. "I think of it basically as an anomaly, because we have had steady growth in ridership over eight years." Transit ridership jumped 42.6 percent here from the subway's 1976 opening through last June 30.

Page and other Metro officials, while saying they lack proof, listed several possible contributors to the decline:

Two big fare increases within six months -- a 22 percent jump last July 1 followed by another 10 percent rise Jan. 1. It was the first time Metro has socked its riders twice in the same fiscal year.

The screeching brakes on the subway, a problem that is just coming under control after a noisy six-month run.

Metro board proposals, which received wide publicity, of major cuts in bus service. Some cuts have been made, but they have been relatively minor.

Problems of general reliability and creature conforts on subways and buses. Too many in-service breakdowns have occurred on both systems, Page's managers say, and methods of keeping subway cars and buses clean, especially on the inside, continue to elude the maintenance yard crews.

Gasoline prices, which skyrocketed a year ago, have stabilized, and some drivers who switched to Metro during the great gasoline crunch of 1979 have returned to their autos.

Metro's complicated fare structure, including the automated fare-collecting equipment on the subway. "I still personally believe our methods of collecting fares cost us riders," Page said.

Metro's ridership projections, based on data from the first nine months, show that the transit system will carry 18.9 million passengers in the 12-month period ending June 30. In the previous year, a record for Metro, there were 187.4 million passengers use both the bus and the subway for one complete trip, they are counted only once.

Bus ridership was projected to decrease this year because of changes in bus service associated with the expansion of the subway's Blue Line to Addison Road. The decrease there is precisely as projected. Subway ridership, however, was supposed to total 83.7 million passengers annually, a 10.6 percent increase of 8 million. Instead, subway ridership is now projected at 78 million, an increase of only 3 percent. The net result in total transit system ridership is a 2.4 percent decrease.

In fact, the subway may do worse than 78 million. In March 1980, for example, the subway carried an average weekday load of 288,000 passengers. In March of this year, the average weekday ridership was 286,000. The decrease appears slight statistically, but in March of this year, Metro was operating three more stations than a year earlier because it had opened the Addison Road section of the Blue Line. Ridership on that section, which serves Benning Road, East Capitol Street and Central Avenue in Prince George's County, has been a major disappointment to Metro officials.

Lower-than-expeted Metro ridership means lower-than-expected revenues. Although Metro had already revised its revenue estimates downward, a further revision may be necessary, some officials said.

Page said yesterday that he does not think the ridership decline should be a factor in Metro's efforts to obtain subway construction money from the Reagan administration. "I believe this is a temporary event," he said.