Sick leave use by longtime federal workers has dropped dramatically since 1968, when Uncle Sam ended a use-it-or-lose-it system and gave workers a pension- boosting incentive to save their leave.

A new study by the General Accounting Office shows that the accumulated sick leave of workers retiring last year was 46 percent greater than those who retired before the incentives were offered. GAO said the average 1968 retiree had 642 hours and "lost" all of it at retirement.

Under the current system, which rewards employes for not taking sick leave, the typical retiree today has 940 hours in his or her account. That leave can be counted as service time.

Use of sick leave overall in government is down slightly, GAO said, but for employes who make a full career out of government service, it is down sharply.

GAO's study, prepared for Sen. Ted Stevens (R-Alaska), could defuse administration proposals to limit the amount of sick leave that employes can store up and apply toward retirement. Stevens is chairman of the Senate's civil service subcommittee and is a backer of the sick leave incentive system.

Government employes get 13 days of sick leave each year. Unlike many private-sector workers, they are allowed to store up leave they don't use. Once they have fulfilled age and service requirements for retirement, they can add accumulated sick leave to their service time as a means of increasing their annuities.

An employe with 30 years of service and 2,080 hours of unused sick leave, for example, can have his or her annuity computed on the basis of 31 years of service, adding about 2 percent to the pension's value.

Based on recommendations of the Grace Commission (the President's Private Sector Survey on Cost Controls) the Reagan administration has sought to change the system allowing sick leave accumulation.

Rep. Ron Packard (R-Calif.) has introduced two bills, one to abolish sick leave credit toward retirement and the other to limit sick leave credit to no more than 50 hours.

Congress ended the use-it-or-lose it system after studies showed that sick leave use was up dramatically in the year before retirement. Some employes took up to six months of sick leave just before leaving, the studies showed.

Administration officials argue that sick leave is intended only for that purpose and that the government shouldn't have to provide incentives for employes not to abuse it. Awards

The National Association of Letter Carriers has been given the President's Voluntary Action Award. The association was honored for its "carrier alert" program, jointly sponsored by the Postal Service. Under the program, people who are sick, disabled or elderly notify their local post offices. Letter carriers then watch for signs of mail buildup or other indications that the resident may be in trouble. Hundreds of lives have been saved in the last few years when carriers monitored their routes, the association said. Overseas Jobs

The CIA is looking for U.S. citizens under 35 who are college graduates for its case officer career development program. Most of the assignments are overseas. For details write: Chief, Career Training Program, P.O. Box 1925-V50, Washington 20013. Job Mart

Federal Energy Regulatory Commission needs a landscape architect, Grade 7 through 11. Call 357-8140.

Agriculture's Economics Management Staff wants two agricultural economists, GM (merit pay) 15. Call Christopher Early, 447-6710.