Washington necktie sellers, whose prosperity depends on the ingrained dress habits of 125,000 government-issue white-collar males, may be in for a summer sales slump. Unless, that is, they wise up and stock safari jackets, tasteful open-neck sport shirts and additional summer dresses. The latter, of course, for our town's 100,000 g-women.

Reason for the above fashion alert: Uncle Sam is going for the throat of the energy crisis, preparing to liberalize office dress codes for the sticky months ahead.

Within the next 10 days the government will begin dropping leaflets (850,000 are in the printing pipeline) on federal desks from the Pentagon to Point Mugu, Calif.

Ever since President Carter decreed indoor summer office temperatures at the 80-degree level, officials and task forces have sought ways to maintain standards and productivity while preventing the government's 2.6 million toilers from melting.

The obvious solution, telling people to dress comfortably, has always flopped because this is, after all, Washington.

Past movements to get people to abandon neckties in summer (Post columnist Bill Gold has advocated it for years) have generally fizzled for lack of high-level support. This is basically a stuffed-shirt town.Many officials want their people to keep their shirts stuffed, no matter what the relatively humidity. Some feared, perhaps with good reason, that once dignified lawyers and professionals would wind up looking like the Beach Boys, rather than serious civil servants.

That was then. This is now.

And soon word will be coming from key federal officials that casual, comfortable summer attire not only is permissible, but also desirable for all save very top brass, chiefs of protocal and the like.

Since high-rankng officials tend to sit near the air-conditioning anyhow, this won't be much of a problem.

General Services Administrator Rowland G. Freeman III, who co-chairs an interagency task force, is the man who will deliver most of the bureaucracy form its sartorial chains. Ladies are invited to take some of it off.

Freeman, a retired Navy admiral, is the man in charge of the heating and cooling of the government's 10,000 buildings. And he has President Carter's ear where federal energy conservation matters are concerned.

GSA is putting out the "Keeping Cool" pamphlets. Much of what they say is little more than common sense. But commom sense is often an uncommon virtue in government, and many managers won't believe it, or do it, unless they get it in writing from somebody higher up.For them, relief is on the way.

"Keeping Cool" will advise civil servants to wear "loose, informal clothing made of fabrics such as open-weave cottons that breathe" so the people wearing them can do the same. It advises male bureaucrats to take off their ties and unbutton collars "whenever people."

For government women, the fashion note for an informal office is "a loose dress or skirt and blouse, sandals and no hose."

At a recent session with his top staff, Freeman said, "I have no problem with people not wearing ties. It won't bother me to see any of you not wearing a tie." More important, he said, "I don't want any of you to say it is okay for some groups, but not your own." Clip and save this, you may need it at your hearing.

Freeman says he will continue to wear a tie, simply because he is the head of the agency and fequently goes to the White House, Capitol Hill and other stuffy (as in formal) places. But for the rest of you G-men and G-women, get ready for federally approved comfort.