Ten percent of the non-government workers in downtown Washington get a fully subsidized parking space from their employers, and 45 percent get some subsidy, according to a study completed recently and believed to be the first report on Washington's downtown private parking patterns ever undertaken on such a scale.

The study, sponsored jointly by the Metropolitan Washington Board of Trade and Council of Governments (COG), was conducted by mail last October among 1,000 companies that belong to the Board of Trade. All the companies were guaranteed anonymity.

The results of the study contrast sharply with the parking subsidy situations of federal employes who work downtown. More than 90 percent of them receive partial subsidies. President Carter announced plans early this year to eliminate most federal parking subsidies here.

Of the 1,000 firms surveyed, 218 responded to the COG-Board of Trade questionnaire. Between them, they have 39,000 employes. The 218 companies all have offices in an area bounded by Capitol Hill on the east, Massachusetts Avenue on the north, Rosslyn on the west and the Mall on the south.

Neither COG nor Board of Trade officials would comment on the study officially, because results have not yet been tabulated or shown to the 218 respondents.

Several major downtown employers who acknowledged receiving the questionnaire and who said they provide subsidies declined to explain to a reporter why they did so.

Comments ranged from "none of your business" (an I Street brokerage firm) to "it's never discussed here" (a K Street consulting firm) to "perks are the name of the game" (a Connecticut Avenue law firm).

The study centered on 6,500 parking spaces. Approximately 5,300 are controlled and allotted by employers directly, usually in lots beneath or beside places of employment. The remaining 1,200 spaces are owned by parking companies, and are leased on a long-term basis or rented by individual employes from day to day.

The study found that subsidies are especially common in what it called the city's core - the "old downtown," centered around 9th and F streets NW, and the "new downtown," centered around Connecticut Avenue and K Street NW.

Ten percent of all private-sector workers in the two downtowns receive full parking subsidies, the study found, and 45 percent receive partial subsidies.

The average cost of a leased parking space in the core is $60 a month, and the average subsidy is $22 a month, the study found. Outside the core, the average cost is $30 and the average subsidy for leased spaces is $9 a month, the study said.

The study also found that a four-year-old Board of Trade program to promote car pooling has apparently failed almost completely.

The program provided information leaflets to banks and other public places, and offered COG's services in matching up employes who wanted to form pools between downtown and a certain suburb. The study found that 16,000 employes at the 218 responding companies had at least received car pool literature.

But only 200 of the 6,500 spaces in the study area were designated as priority spaces for pool cars. And not all 200 of those spaces were being used every weekday, according to the study.

The study also found that only 1.2 persons are riding in the average car on a downtown street at midday. That is one of the smallest such average figures in the metropolitan area, according to traffic studies conducted last year by the U.S. Department of Transportation.

George Marcou, manager of the Board of Trade, said the study is expected to attract special attention because of President Carter's announced intention to force federal employes to pay for parking.

"Our motive for the study is,'What can the private sector in downtown do to encourage use of public transportation?'" Marcou said. "Now we know."