The recent retrospective look from the 1940s is a thing of the past. The fashion world has taken a quamtum leap into the 1950s.

One wonders what the designers might be doing if the movie "Grease" never opened in Paris. But it did. And served to endorse an already popular dressed from the fleat market.

If you can strapless dresses and circle skirts, cinched waistlines and pedal pushers, Harlequin glasses and bobby socks, you get a sense of the fashion message being stirred up in Paris for the coming spring and summer season, as interpreted by the French ready-to-wear designers.

Even the Paradise Latin, the popular cabaret-nightclub with a sense of humor, has changed its opening number to a "Grease" take-off with Travolta and Newton-John look-alikes in black leather pants.

The current crop of Paris designers who are into the '50s look weren't around way back then, so when they take all the elements of that period, they turn them into modern, witty and probably even saleable clothes.

To be sure, "Grease" is not the only show in town. Running has become a snob activity and the Bois de Boulogne a popular early-morning track, so jogging outfits, athletic shorts and sweat suits are in many collections. Of course, here they are done in marvelous textures of all cotton, in unexpected bright colors and patterns in contrast to the polyester variety more readily available at home.

Also there is an important silhouette that is apparent. It starts with broad shoulders, tapers to a cinched waist, then rounds off over the hips to a slim skirt or tight pants. Karl Lagerfeld, the fashion business's intellectual, calls it "morphologic graphics" and sketches out a triangle over an oval to make the point.

"Clothes have gotten too messy; people could put any shape under those tents. So it became necessary to change to clean, graphic lines," says Lagerfeld, who has turned in his own favorite costume of the past couple of years - a silk or cotton loose-fitting overshirt for a white starched collar shirt, blue satin narrow tie, and vest and jacket in matching raspberry colors. ("I'm tired of all the dark colors. I felt it was time for some bright ones," he said.)

Like the other designers. Lagerfeld's use of simple, graphic shapes, like we last saw in the 1950s has a thoroughly modern interpretation. His strapless tops, for example, are boned yet molded in foam to give them shape and help them stay up. No push-up bras or merry widows this time around. And Lagerfeld isn't risking what happened on the runway in Milan, where a strapless top slipped off a skinny model, who simply continued to wear it that way.

For many designers the new silhouette calls for a hemline that is shorter than that just beginning to catch on in Washington. Skirts frequently are cut off just below the knee, not an impractical idea when they are narrow. It's easier to walk that way. But there are still plenty of longer ones around, and shorter ones that look just fine for young girls who like to show off their legs on a hot summer day.

Bermuda shorts are likely to show up in some offices with cotton blazers and a triped T-shirt under, and it should be just as appropriate, even more perhaps, than jeans.

One established length is for pants that fit widely (and roundly) over the hips and taper to a tight fit, always at the ankle or shorter. And then for those who like the lengths in between, there are Capri pants (mid-calf), pedal pushers (below the knee), Bermudas (two inches above), Jamaicas (two inches shorter than Bermudas), and no one needs to be told about athletic shorts, which are around too.

And the big circle skirt in its last incarnation was never worn unbuttoned to the crotch with a black leotard showing underneath, as France Andrevie showed this week. What is being repeated is the favorite lunge costume of the '50s, a button-front dress, worn open below the waist over tight-fitting toreadors. Only now it's designed to wear out on the street or to a disco.

Buyers arriving from the Milan showings were well-prepared for Paris' most unwelcome aspect, due largely in part to the declining dollar value. Last season a carnet in the Metro (10 tickets) cost 10 francs (just over $2). But now with the small price increase and the exchange difference, the same ticket cost $3. Two New York buyers breezed out of their hotel to a neighborhood dinner last night and got a bill for $60. Parisians consider a fixed price dinner at 50 francs almost a cheap meal - at today's exchange that's roughly $12.50. Such restaurants, which not much more than a year ago had fixed price meals at half that, are thriving.

Cleary an effort has been made to get the sows rolling more smoothly this season. Kenzo is showing only to small groups by appointment. And several designers - Chantal Thomass, Claude Montana and Castelbajac - teamed up to have their shows at the same location away from the center of town, giving out the same assigned seats for all the shows. Only one problem: There are showings in other parts of town between these shows, so having all the shows in the same place doesn't really help that much.

Designers have scouted out appropriate places for their '50s reruns. Yesterday morning, for example, the Ecriture showing was held in a high school gym; a crooner mouthed "Smoke Gets in Your Eyes." Several models wore pompadours, and Lucilla Bali-like scarves wrapped heads. A Dixieland band marched on, and others danced the Lindy.