Sen. Lawton Chiles (D-Fla.) said yesterday he expects to give up the chairmanship of the Senate District Appropriations subcommittee which must act on the city's budget each year.

It is unclear who will succeed Chiles on the locally-crucial subcommittee but it is likely that the new chairman will be one of the 18 new senators who have jutst taken office.

Chiles plans to give up the role he has played during the past two years of asking tough questions about the District's spending and revenue-collection to move over to the chairmanship of the Appropriations Subcommittee for Treasury, the U.S. Postal Services and General Government.

The chairmanship of that subcommittee became vacant with the defeat in November of Sgn. Joseph M. Montoya (D-N.M.). Chiles is in line through seniority to replace Montoya. The Florida Democrat said yesterday he expects to surrender the District subcommittee chairmanship in order to do so.

A freshman senator is likely to head the District Appropriations subcommittee because each of the veteran members of the full committee is expected to be occupied with heading other subcommittees.

Freshman Sens. Donald W. Riegle (D-Mich.), Paul S. Sarbanes (D-Md.), John Meleher (D-Mont.) and Dennis DeConcini (D-Ariz.) have been chosen by lot to serve temporarily on the Appropriations Committee until permanent committee assignments are made following action on a proposed reorganization of the Senate committee structure.

Should they remain on the powerful Appropriations Committee permanently, Riegle, who ranks ahead of the other newcomers in seniority, or Sarbanes, who as senator from a neighboring state has a local interest in the District, might be good bets for becoming though the subcommittee chairmanship - one of the Capitol's chief watchdogs over District spending.

However, the new senators temporarily on Appropriations are not necessarily more likely than any of their freshmen colleagues to gain the coveted permanent assignments to it. And a new District subcommittee chairman is not likely to be named until after the permanent committee members are chosen.

Mayor Walter E. Washington yesterday said, "I appreciate Sen. Chiles' efforts on behalf of the District of Columbia and I wish him well in his new assignments."

In his two years as the Senate's Chief reviewer of District budgets, Chiles developed a reputation as one who did not always accept easy answers from city officials testifying before him and one who pushed hard to reform the city government's fiscal management.

Chiles has been a severe critic of the management of the District's Department of Human Resources. Joseph P. Yeldell, former DHR head, appeared before Chiles' subcommittee in September to complain. "As long as a program is running fine, nobody talks to our people and certainly nobody gives me any credit. Let one thing go wrong, and you go back to me, management, waning to know what is wrong."

Chiles replied, "If it was one thing, Mr. Yendell, I might agree with you, but it is one thing after another."

Through Chiles' efforts, an extra $3.2 million was added to the District budget in September to provide for 188 additional employes at Forest Haven, the city's understaffed facility for the mentally retarded.

Chiles also has been pressing District officials to raise the Metrobus fare from 40 to 50 cents during rush hours to help meet Metro's operationg deficit.