"If I work 24 hours a day, it's OK with me," says Jon Giswold. "I'm in love with this profession!"

The object of Giswold's affection: fashion - specifically, menswear designing. Just in his 20s, and in business for himself for about two years now, he's generally regarded as one of the rising stars in the industry. He's also consideredone of the innovators who's challenging tradition and providing men with interesting and more creative ways to dress."There are a few of us in this business - Bill Robinson, Ronaldus Shamask - who are eager to chart new horizons," Giswold says. "We're trying new, bolder silhouettes; we're stretching the limits of what's acceptable. It's exciting work, and it's something I've always wanted to do."

When he was growing up in the little town of Grantsburg, Wis., Giswold used to dream of becoming part of the glittering world of fashion. Or maybe becoming an actor. Of course, what he really wanted was a chance to combine the two - acting and designing.

He began working toward this goal with a job in retailing.

"I listened carefully to my customers," Giswold recalls. "I talked with them about the kind of clothes they needed and wanted, and I discovered that most men were hunting for smart, quality clothing that would take them from the office out to dinner and the theater in style. Versatility was very important."

The young designer also learned quality and investment dressing were top priorities. His customers told him again and again that they didn't mind paying good money for apparel - but the apparel had to be worth it.

Armed with such information, and eager to learn still more about the rag trade, Giswold soon headed for New York City's garment district. He landed a job with Alexander Julian, the menswear designer who's known for his creative way with color. In the couture salesroom, the aspiring designer quickly got an education in fabric purchasing and production - essential knowledge, he says, for anyone who hopes to have his own firm someday.

A bit later, the young man from Wisconsin tried his hand at free-lance designing, working for such established names in the trade as Mary Jane Marcasiano and doing clothes in his spare time for private customers. The private clientele included both men and women and some pretty prestigious names, Giswold points out with pride. Actress Lucie Arnaz, for one.

Finally, he managed to raise enough capital to launch his own label and open a small menswear firm. It was a big gamble, he admits - especially for someone who had no formal training in the field. And there have been plenty of shaky times and disappointments since. But there also has been recognition and a measure of success.

"I feel fortunate to have made it at all," Giswold says. "This industry's such a tough one. Even the real biggies like Stephen Sprouse come and go. It takes a tremendous amount of capital to keep your doors open. Then there are the problems related to manufacturing and shipping. When you think about all the stuff involved, it's mind boggling."

Although it may be mind boggling to try to make it in fashion today, it's a challenge - and one the young designer enjoys. He also says that mistakes can be the best teachers. One thing he has learned: separates sell well in the menswear market right now. And so, instead of focusing on complete ensembles the way he did at first, his firm's now turning out mix and matchable pieces that give the customer the chance to be creative and do a bit of designing himself.

When the firm isn't occupying his time, Giswold likes to do seminars at the Fashion Institute of Technology and tell the students about his approach to designing and being an entrepreneur.

On designing: "I'm pushing at convention and trying new things. For fall, there are sport coats with suppressed waists and accentuated shoulders. Clothes that echo the elegance and style of Gary Cooper - I've even named one of my jacket silhouettes after him. The pants have pleats - there's an ease to the way they fit. My palette's simple: Black, gray, camel and winter white. Patterns are simple, too, but dramatic - checks, plaids. As for fabrics, only the best and things that have a vintage look."

On being an entrepreneur: "I tell the kids it isn't going to be easy. Maybe a few will fall into good jobs or be able to have their own labels and become household names like Calvin Klein. But most will struggle and have to sweep floors to keep things going and pay the rent.

"When I started out, my little apartment doubled as office and showroom. Even now my office is very small and there isn't anything fancy about the operation. I do almost everything myself, with the help of a couple of FIT interns and a bookkeeper. And will I still be in business tomorrow? Well, who knows? It's a precarious existence, let me tell you, and I think kids just starting out should be prepared. I'll tell you one thing though. I'm giving it my best shot!"

If the best shot misses, Jon Giswold will be saddened but not devastated. He has a plan.

Menswear for seven years. If he hasn't made it by then, and if his private designing for show business personalities hasn't gone anywhere, he will simply put a "closed" sign on the door and start again.

Giswold will be 35 in seven years - still plenty young enough to launch a new career. And what will it be?

Public relations, probably. But PR for the fashion industry, not anything else. He loves the apparel business far too much to ever leave it behind completely.