The number of rush-hour commuters heading into central employment areas of Washington and Northern Virginia by car, subway and bus has increased 6 percent in the last two years, the sharpest rise since the mid-1970s, according to a new survey by the Metropolitan Washington Council of Governments.

The recent increases have occurred mainly in car and van pools and in public transportation, chiefly the Metro subway system. The number of commuters driving alone at rush hour has edged up only slightly since 1983.

The survey, which was released in draft form at a COG committee meeting yesterday, encompassed most of the region's central employment area, including the District's downtown, Capitol Hill and Georgetown, along with Rosslyn, Crystal City and the Pentagon.

The increases in rush-hour trips appear to stem largely from growth in employment, officials said. The report pointed to "vast construction of office buildings and an increase in employment opportunities in Rosslyn, Pentagon City and Crystal City."

Christopher R. Neumann, a Council of Governments official who oversaw the study, said the relatively slight increase in commuters driving alone may be attributed to several factors. These include mounting traffic congestion, rising parking fees, expansion of the subway system and car pool rules on some highways.

The survey, compiled in March, April and May, found that the number of persons traveling into the region's central business areas between 6:30 a.m. and 9:30 a.m. on a typical weekday climbed from about 413,000 in 1983 to nearly 438,000 this year.

The increase was larger than in any two-year period since the council began surveying rush-hour trends in the mid-1970s.

The number of persons driving alone rose by only 2 percent. An earlier COG survey reported a 15 percent increase in single-occupant trips between 1981 and 1983.

The number of commuters in cars carrying at least two persons rose by 5 percent. The report attributed the rise partly to a sharp increase in traffic on I-66, where car pool rules were eased in January 1984.

Under the congressionally mandated shift, the heavily used section of I-66 inside the Capital Beltway was opened to car pools carrying a minimum of three persons, instead of four. The survey said the number of cars on I-66 inside the Beltway at rush hour more than doubled from about 3,300 in 1983 to about 7,600 this year.

The report cited an 11 percent increase in the number of commuters using public transportation, including the Metro bus and subway system along with other commuter bus and rail services. The gains were linked partly to the opening in recent years of subway extensions in Montgomery and Fairfax counties.

Metro officials reported recently that mass transit ridership has climbed to its highest level in 30 years, reaching a mark last recorded at the end of the Korean War when the region was served by privately owned bus and streetcar companies.

Ridership on the Metro system surged during the nationwide gasoline crisis in 1979 and 1980 but dropped during the early 1980s. The trend was reversed in mid-1983. Metro officials have attributed recent increases to improvements in service and reduced unemployment along with expansion of the rail system.

According to COG's surveys, the number of commuters in cars declined at the time of the gasoline shortages, but the downward trend ended in 1982.

The new study found that rush-hour traffic rose more rapidly on Northern Virginia highways than on roads in the District, where increases of only about 1 percent were recorded from 1983 to this year.

The finding appeared to be in line with a recent survey by D.C. officials that reported a decrease in rush-hour congestion in the central part of the Washington area since the mid-1970s. They attributed this mainly to a decrease in the number of Metrobuses on city streets.