Think of a spa these days, and you probably picture a health club on a suburban highway where young and not-so-young upscale professionals go to work out several times a week.

But for traditionalists, a spa means something quite different -- a place that is part hotel, part convalescent home, part spiritual retreat, a place where people go for a week or two, or even for several months, to lose weight, to change their health habits and to escape from the daily grind.

This kind of spa evokes images from the past: the rose gardens of Elizabeth Arden's posh Maine Chance Farm where the heiresses and movie stars of the '40s went to escape from the limelight; the linden-lined promenades of European spas like Baden-Baden and Montecatini Terme, where the aristocracy of Europe went to take the waters, and to see and be seen.

The grand old days of the spa may have faded -- the original Maine Chance is long gone (although its successor, the Maine Chance in Phoenix, Ariz., is thriving), and the European spas have become the province of stout matrons who seek the cure for their arthritis and dyspepsia -- but they are not gone. In fact, spas are making a comeback -- in the United States, anyway -- thanks to the extraordinary growth of interest in physical fitness.

Most United States spas are located in the Sunbelt states, where climate makes operating a spa more economically feasible. But the Northeast has one posh spa of its own, Gurney's Inn in Montauk, N.Y., where you can be wrapped in herbs, packed in mud, pummeled, massaged and whirlpooled to your heart's content. The prices are not cheap, to be sure, but they are nevertheless a lot less than the $2,000 or more a week charged by some of the West Coast spas.

And Gurney's is unique in being one of the few spas in the world to offer thalassotherapy (thalassa is Greek for the sea) or sea-water treatments. Set on a cliffside overlooking a magnificent stretch of white sand beach, Gurney's draws on its seaside location, with sea water and seaweed figuring prominently in the treatments. "No one would need a psychiatrist if they lived by the sea," says owner Nick Monte, who operated Gurney's as an inn for 26 years before fulfilling his dream of adding a spa six years ago.

My two-day visit to Gurney's began with an introduction to the Baroness Hildegarde von Menklenberg, of Heidelberg, West Germany, whose title is "spa consultant." The Baroness, as she is called, has the enviable job of visiting European spas in search of new therapies to introduce at Gurney's. She said, for instance, that Gurney's seaweed treatments were imported from a thalassotherapy spa in Brittany, France. Many European treatments, however, aren't suited to American spas, she explained: American spas emphasize beauty and fitness, while European spas concentrate on physical therapy. "No American woman would be happy at a European spa," she said.

With that, she escorted me to the locker room, where I was issued a towel, a key and a salmon-colored terry kimono. I was ready for rejuvenation.

My first treatment was a "brush and tone," in which I was sanded down, so to speak, with a natural-bristle brush, all the while listening to gossip about Gurney's many famous guests -- who have included Mary Tyler Moore, Alan Alda, Cheryl Tiegs and Lauren Bacall. I was reassured to learn that even the stars "really don't look all that good" when scrutinized at close quarters.

Then I was massaged with seaweed "milk" imported from Saint-Malo, France -- a milk that supposedly contained amino acids, phytohormones and vitamins to improve the tone of my skin. (The next day I would have a similar treatment, a "seaweed cell fluid wrap" in which I was anointed with a seaweed extract and then wrapped like a mummy in linen sheets. In both cases, I was skeptical of the pseudo-scientific jargon, but I emerged with skin as silky as a newborn's. To my amazement, it stayed that way for a week or more.)

My next therapy was fango, not a dance but mud therapy (fango is Italian for "mud") using volcanic mud imported from the hot springs of Battaglia Terme, Italy. ("You sure it wasn't Long Island Sound?" snickered my husband.)

The mud is heated, mixed with paraffin and poured into large trays lined with plastic. The plastic sheets of mud, which look like cafeteria trays of brownies, can then be cut to fit different parts of the body. The therapist asked which of my muscles tend to accumulate the most tension, then applied the hot mud to my back as I lay face down on a massage table.

Expecting that to be the extent of the treatment, I soon learned there were more pleasures in store: While I was relaxing under the pack of soothing mud, the therapist turned her attention to my feet; for 20 minutes, she massaged my feet and calves with warm almond oil. This was heaven.

At home later that week, I read up on fango therapy at a medical library. I learned that it has been in use since Roman times as a therapy for arthritis and other aches and pains; mud, it seems, has great heat retention properties.

Gurney's boasts that its imported mud contains beneficial minerals that are absorbed by the skin; but, according to the medical literature, it is the heat itself that confers most of the benefit. And it did feel wonderful.

The fango therapy was followed by a low-calorie lunch of an artichoke, three shrimp and half a baked apple (for those who are not dieting, a regular menu is available in another dining room).

Later in the afternoon I took advantage of some of the exercise classes that are offered throughout the day, including yoga, aerobics and aquatic exercises in the huge glass-enclosed, saltwater pool overlooking the beach. There I had a chance to meet some of the other guests, who, despite Gurney's glamorous image, seemed to be regular folks.

One woman had been given the four-day mini-week for her fortieth birthday by her family. Another woman, the head of her own insurance consulting firm, was treating herself to her first vacation in seven years. Still another was a spa groupie who seemed pleased at getting treatments similar to those offered at the West Coast spas at a fraction of the cost. (The majority of Gurney's guests are female, but a "Five-Day Special Health, Fitness and Optimal Performance Plan" has been successful in attracting male executives.)

At 5:30, I checked in for my final treatment of the day, thalassotherapy, which takes place in a huge turquoise plastic bathtub equipped with countless jets. The tub is filled with warm sea water, which is agitated in a Jacuzzi-like effect. Then the jets are turned off, and a therapist administers an underwater massage with a stream of water from a high velocity hose.

The massage left me feeling totally relaxed -- the perfect way to end a tough day at the spa. Returning to my room later in the evening, I found a care package on my pillow -- a Gurney's tote bag containing hot-air-popped popcorn, mineral water and a tape measure to measure those shrinking inches. I ate the popcorn on the balcony of my room overlooking the Atlantic, whose waters gleamed in the light of a full moon.

I left the next morning, after my second seaweed treatment. But I didn't want to. The day had begun with an aerobic beach walk preceded by warm-up exercises on a sunny beach-front deck. "I can't hear what he's saying," said the woman next to me as the voice of our young leader was drowned out by the roar of the surf.

"But who cares?" she continued. "He's cute, the sun's shining, and I'm at Gurney's." She put it perfectly.