Rationing will be imposed this summer on federal departments and agencies wanting to add more political appointees to top-level posts in government.

The demand for more noncareer appointees under the Senior Executive Service already has exceeded available legal supply levels. The Office of Personnel Management, which controls the legal mix of 90 career executives for every 10 political appointees in the SES, says most agencies will have to make do with their current allocation of top-paid executives who serve at the pleasure of agency heads.

Under the SES, part of President Carter's civil service reform, between 8,000 and 9,000 Grade 16 through 18 government jobs gradually will be coverted to a system offering less tenure and security in return for the chance to earn higher pay, bonuses, better retirement and paid sabbaticals.

Incumbent "supergraders" (about 80 percent of them in metro Washington) will be offered the chance to join the SES this July. New appointees must come into it.

By law the number of jobs set aside for noncareer (political) appointees in the SES cannot exceed 10 percent of the total SES membership. OPM has formally advised agencies that the number of noncareer assignments already authorized is 98 percent.

Since OPM wants to have a reserve or "pool" of undersignated political jobs for emergency assignment, the prospects are that few agencies will be allowed to increase their non-career work force. Those who do get more political jobs will have to take them away from other agencies to maintain the 90-10 mix.