When the Cincinnati Bengals creamed the San Diego Chargers in Cincinnati last Sunday in one of the coldest games ever -- wind chill minus 59 -- both teams were wearing long underwear. So was much of the crowd. For good reason.

"The most important layer is the first layer," says Dr. Murray Hamlet, director of the Experimental Pathology Division and Cold Research of the Army's Research Institute of Environmental Medicine.

While the Bengals wear every variety, many amateur athletes are becoming selective about their long johns. The old red and itchy union suit has evolved into a sophisticated and scientifically-engineered range of garments. Some of the designs are so attractive that they're apt to show up as outer -- rather than under -- wear in the poshest ski lodges. Just as the growing numbers of cross-country skiers, runners and back-packers have demanded the best in equipment and gear, they're seeking the same refinement in what they wear under it all.

The essential thing in winter is keeping the skin dry, and the synthetic fiber polypropylene does it best, says "Hamlet. "It transfers the moisture and the skin stays dry. And it is very durable. If the moisture is moved away from the body and the evaporative cooling occurs some distance from the skin, the clothing feels warm and the skin stays dry."

Henry Barksdale, an owner of Moss Brown & Co., Georgetown specialists in running and outdoor gear, first learned of the Norwegian-made Lifa body wear -- developed for Norway's cross-country ski team -- from a friend who went cross-country skiing in Norway three years ago.

Barksdale, who has been running this week in freezing temperatures, wears a Lifa top and bottom and a Gore-tex running suit. He recommends Lifa as strictly for active sports.

The warm Thermolactyl women's slip, which originated in Europe and is available in this country, is starting to sell so well the company is about to introduce camisoles, half slips and nightgowns in the same synthetic fabric, according to marketing director Frank Bussone.

Jeffery Pinkard, specialist in warm underwear at Washington's Eddie Bauer store, finds little difference in the several brands of synthetic underwear the store sells.

"Some have racing stripes and some are thicker material than others, but (those factors) don't seem to make a real difference.

"However, some people just can't deal with synthetics," says Pinkard, who advises those customers to try cotton fishnet underwear.

Wool also wicks moisture away from the skin, although not as well as sythetics, according to Hamlet. Wool undergarments were, of course, standard wintergear before central heating, and there are still many women who will never give up their "woolies." With the energy crisis and lower home and office temperatures, underwear -- of some variety -- has become essential for many men and women.

"Wool is most people's concept of warm underwear," says Sandra Goldenberg of Hanro, whose factories in Switzerland make some of the finest silk, cotton and wool-knit under wear. Wool is the best seller in winter, says Goldenberg, partly because CAPTION: Illustration, "Please wrap it as a gift." Drawing by Mary Petty; (c) 1938, 1966 The New Yorker Magazine, Inc.