Four of the biggest names in America's $126 billion-a-year fashion industry last night debated some ever-debatable questions whose answers keep them all in business and women in clothes: What is fashion, where is it coming from and where is it going?

"It's easier for men to design for women because we can be more objective," said Oscar de la Renta, whose trademark is romantic fantasy clothes.

"Oscar, you see women like this," scolded Pearl Nipon, the creative half of the firm called Albert Nipon, as she defined a curvy silhouette with a wiggle of her hands, "but they're more like me.Women mature with age -- they're built like pears."

De la Renta, his expressive Latin

Temperament perfectly under control, replied brightly: "I don't know any pears."

This friendly sparring reflected the point-counterpoint tone of the evening as De la Renta and Nipon joined Bill Blass and Donna Karan, who designs for Anne Klein, before a sellout audience of 800 paying $5 each at a symposium sponsored by The Washington Post to benefit the Hospital for Sick Children.

Held in Georgetown University's Gaston Hall and moderated by Post fasion editor Nina Hyde, the event turned into a wide-ranging discussion touching on such things as why clothes cost so much, whether designers copy each other and how fads like jeans get started.

Designer jeans, said De la Renta, are "a way of life, the one American design adopted by the whole world -- we live by fads, and today, they are designer jeans."

According to Karan, the jeans market is headed towards less expensive sportswear under designer labels. "What made Gloria Vanderbilt and Jordache so expensive is the promotion -- television's expensive."

"Most jeans cost us about $9 a pair," agreed De la Renta.

"What I don't understand about designer jeans," said Nipon, whose firm doesn't make them, "is why anybody wants to wear anybody's name on their backside."

"Enough of jeans," said Blass, who name also appears on such things as cars and furs who drew the line at designer caskets or braces for the teeth.

Looking into his crystal ball of fashion, De La Renta predicted softer, less mannish clothes ahead for women, while Karan talked about greater "communication" between American designers and their customers.

European and American designers may be "playing around in the same fabric mills," Karan said, joining the chorus of her colleagues denying they are influenced by European designers. 'We're working with our customers," she said. "In Europe they tell women what to wear." w

Blass, who wants something "downright sexy" in each of his collections, said that "despite women's lib, I still believe women dress for men." For inspiration, he said, he thinks of "a composite of a dozen gals all over the country."

When it comes to men and who should design their clothes, however, Blass was adamant. "Being a man, it's easier to design men's fashions," he said. "I don't think it's easier for women to design women's fashion."

Other topics under discussion, some prompted by questions from the audience:

Discount stores -- "We have to have outlets for our damaged merchandise and our overcuts, but none of it is sold there at the beginning of a season," said Nipon, who started out in Philadelphia (where she and her husband still live) designing maternity clothes and whose best known trademark today is the pleat."A pet saying we have in the business is 'Would you build a mansion without a bathroom?'"

The preppie look -- "Except for the freaked-out '60s, it never really left us," said Blass. "It's really an American look."

High price tags -- "We're embarrassed to ask you to pay these kind of prices," said Karan. "But wardrobe building is something you do over the years. We're not saying go out and buy a complete one from any of us."

Hemlines -- "No prescribed hemline," said Blass. ("It's not an issue anymore," De la Renta said earlier, describing any audience that asks about it as "a bad one -- a good one wants to know what direction fashion is going.")

Welcoming the crowd were Post publisher Donald Graham and Georgetown University's president, the Rev. Timothy S. Healy. Since the subject was fashion, Healy pointed out that the design of his own clerical shirt was "at least 150 years old." Which seemed to refute Pearl Nipon.

"The one thing constant about fashion," she said, "is change."