The 75,000 federal workers now on the four-day week will have to return to the Monday through Friday grind unless Congress pumps new life into Uncle Sam's about-to-expire compressed work-week program.

That experiment, which also allows another 100,000 to work a four-day week every other week and lets 150,000 more set their own hours (within reason) is due to end March 29.

The three-year test allowed and encouraged agencies to waive the rule governing overtime after eight hours a day if employes volunteered to work longer days so they could put in shorter weeks.

Nationwide 325,000 government employes, from clerks at the Geological Survey to radio monitors at the National Security Agency, are on compressed or alternate work schedules of some kind or other.

Variations of the eight-hour day allow employes (75,000 by best estimate) to put in a 10-hour day four days a week. Other options allow employes to work longer hours over a two-week period so they get a three-day weekend twice a month. Still others allow employes to come in early (or late) and leave early (or late) or to work short hours one day, longer hours the next so long as they put in a 40 hour week.

Workers generally like the system. It is voluntary, and allows people to schedule things like doctors' appointments, license renewals or personal business without charge to annual leave (vacation) time. Some supervisors are not so crazy about the system--most of them are not on it--because they say scheduling and policing the system takes too much time.

The Reagan administration -- through the Office of Personnel Management -- favors the system, but wants it tightened up so that management will have greater control over who is in it.

The House Post Office-Civil Service Committee, which rarely gets unanimous agreement on anything, passed without dissent a bill extending the program. It still must clear the full House, then go through the Senate, which takes time.

Agencies here using the four-day week for some employes include Treasury Department, FBI, Bureau of Engraving and Printing and the Coast Guard.

If the experiment is continued, the number of federal workers -- and employes of related private industries that deal with the government-- on the four-day week could easily double in the next few years. It opens up the possibility of agencies providing 10- or 12-hour service (so citizens can conduct business before or after they go to work) and Saturday openings, too, if necessary.