First off, let the record show that I'm the last one to criticize other people's search for physical fitness. Sit-ups on the bedroom floor in the morning may not do anything for your middle--some ambitions, I've concluded, should be retired--but it can do wonders for your morale. You can exercise in the morning and spend the rest of the day feeling infinitely superior to everyone else.

Physical fitness seized America some years ago--back, it is important to note here, in more affluent times. Before you knew it, perfectly sensible people were running around their neighborhoods in expensive Italian shoes and designer shorts. Cocktails were out, Nautilus in.

Soon the craze hit the Senate, and it has apparently never fully recovered. The same Senate that last year cut 5 million doses out of the federal child immunization program in order to save money is about to furnish itself with a physical fitness gym, a $736,000 monument to special privilege.

The Senate approved plans for a gymnasium back in the mid-'70s at the beginning of construction of the Hart office building. By the late 1970s, however, cooler heads prevailed. Out went, among other things, the gym. Time passed. Then one of the most amazing things in the annals of public works happened: with the building almost finished, there was money left over.

The Senate Office Building Commission, which is composed of five senators, met. Back went the gym. Back also, according to Sen. William Proxmire (D-Wis.), went more than $3 million for other "frivolous, nonessential projects."

This was too much for Proxmire, who is not only a ranking member of the Senate Appropriations Committee, but is also one of Washington's ranking joggers. "Senators," he pointed out this week, "already have two gyms, one of which is rarely used . . . Most of us are physically inert, tired, beat, old codgers for whom a workout means stretching out on a table to absorb a rub-down."

Elliott Carroll, executive assistant to the Capitol architect, disputes at least a portion of this assessment. Senators are currently toning their muscles in two small rooms that could hardly be called gyms, he says.

"They are 15-by-24-foot rooms where they have exercise machines and two small swimming pools that are practically bathtubs, really. They are quite old and in the basement of the Russell Senate office building."

Plans for the Hart building call for a facility the size of a junior high school gym where members can not only exercise, but also play basketball and volleyball. It will also be open to the Senate staff, which is at least more egalitarian than the other gyms, which are open only to members of Congress.

"There is strong feeling among the senators that voted for it that good health is promoted by exercise," says Carroll. "They are often held here long hours and long days. It's a matter of good health maintenance."

In an ideal world, senators could undoubtedly benefit from such a gym. But this is not, as the Senate has been busily reminding the public, an ideal world.

The same Senate that is anxious about its own health cut the child immunization program by 10 percent at the time that costs of inoculations are going up 30 to 40 percent. Despite warnings of probable polio epidemics, the Senate allowed 2 million youngsters to lose out on immunization this year.

This Senate tried (and thanks to the House failed) to reduce venereal disease programs by 25 percent despite severe increases in venereal disease. It declined to provide $9 million that the Center for Disease Control wanted to use for controlling TB, despite the fact that some 15 million Americans are infected by that disease. CDC did not get a $2 million program for genital herpes surveillance, despite its contention that there has been an epidemic of this virus in the country since 1966.

These, too, are matters of good health maintenance. For the thousand newborns exposed this year to herpes, these are matters of death, blindness and retardation.

Proxmire, says an aide, thinks the money for the gym and other non-necessities ought to go back to the Treasury. This, of course, is unheard of. But these are hard times, and it might be that the senators could tighten their belts along with the people whose belts they've been tightening. At $2 a shot, the administration's figure, the $736,000 for the gym could immunize more than 350,000 children.

There is, after all, nothing wrong with senators doing push-ups in their bedrooms in the morning just like the rest of us. Rank has its privileges, but this is not the time to flaunt them.