HAROLD PRINCE was window-shopping on Lexington Avenue in New York last year when he felt a tap on the shoulder and someone asked,, "Are you still alive?"

The voice belonged to Prince's cardiologist, whom he hadn't seen in five years, not since the doctor had said he would die if he didn't agree to coronary bypass surgery.

Prince didn't, and he isn't.

Thanks says the writer/editor, not the playwright/director of the same name, to a radical change in his diet, which his wife Francine created.

Prince's story begines, not six years ago, but when he was in his 30s. "I was a very fat man. I weighed 180 pounds and I'm 5-foot-4. I went on a high-protein diet, which was really a high-fat diet, but we didn't know that then. We didn't know it wasn't good for you. I lost 50 pounds one summer and kept it off, but essentially I stayed on a high-protein, high-fat diet."

He stayed on it through all European gastronomic tours with his wife ("People go and look at churches; we'd look at the local deli. We ate our way through Europe."), through all their restaurant dining in this country, through all their entertaining.

francine Prince, once a singer and a designer, has always loved to cook, especially rich French food. "I was born," she says, "to be married and to be in the kitchen." The Princes have been married for 17 years. It is a second marriage for both of them.

Prince is certain his dietary habits caused his massive heart attack. All three artieries were clogged and the doctor didn't expect him to live. "I wound up all my affairs. We didn't renew the lease on our apartment." As a result, Prince noted with a certain amount of chagrin, "we're paying through the nose."

The doctor recommended a bypass, but Prince, who is a medical writer along with his many other freelance writing and editing efforts, knew the benefits and the risks involved, and vetoed the idea. Instead, he told the doctor, he would try diet as a cure.

The doctor threw up his hands. His only advice to Prince was to "cut back a little on the salt." He also told him: "If you walk around the block, you'll drop dead."

For four months Prince, the gourmet, endured a "careful, tasteless diet." And he suffered. But he says, "People who go on resticted diets feel they have to suffer." Just the same, he was about to abanbon his low-fat, low-salt, low-cholesterol regime, even if it killed him.

His wife, however, wasn't prepared to give up without a fight. So she began experimenting. Her formula was based on a need to prepare food which wouldn't make those who ate it feel deprived. For her husband it has worked. "I do think if you eat well your emotional attitudes change. I'm more tolerant of other people; I'm more gregarious. If you're not enjoying what you're eating, it's not fun."

And Prince, who confesses to 64 when pushed, but only because no one believes it, says he is enjoying life to the hilt. "All I know is that I'm very healthy. I haven't had an angina pain in years. I work 10 hours a day."

He also walks four miles a day. It took about six months for some improvement to show. "After two years I was 80 percent better, now I'm gung-ho." So gung-ho that he's climbed a mountain.

"The truth is," Prince said, "It would never have happened except for Fran," who eats exactly as her husband does. "She had to extend herself. She had to be inventive."

Her inventions, created in a Manhattan apartment-size kitchen, are now between the covers of a book called "The Dieter's Gourmet Cookbook" (Cornerstone Library, $4.95)9 All of the recipes are low in fat and cholesterol and use no added sugar or salt. Each recipe is accompanied by a per-serving breakdown of calories, fat, polyunsaturated-to-saturated-fat ratio, sodium, carbohydrates and cholesterol. Unlike so many recent cookbooks which advertise themselves as sugar-free, this one really means it. Francine Prince has not substituted honey or fructose or saccharin for sucrose. Sweetening in desserts is derived from the natural sweetness of the ingredients.

She takes some pains to explain not only how the recipes were developed, but how to use the book: what is forbidden, what is allowed; where certain specialty items, of which there are few, may be found; how to make the basics. Wine, many people will be happy to know, is an ingredient in a number of recipes, and the Princes continue to enjoy it with their meals. The good taste of the food is derived from the generous use of herbs, spices, especially garlic and a couple of special seasonings available at natural food stores.

Their diet is 25 percent fat, 15 percent protein and 60 percent carbohydrates. "Nowhere near as stringent as Pritikin," Prince hastens to point out.

Harold Prince, who was the ghost writer of what was probably the first popular book to deal with the relationship between diet and coronary heart disease, based on Dr. Norman Jolliffe's Prudent Diet, describes himself as "the Eisenhower of nutritionists. I just don't want to take any chances." Because of this attitude he supplements his diet with vitamins. "Food is depleted of vitamins because of transportation, storage, etc.," he explained. "You get sufficient amounts of minerals in what you eat because they don't deplete."

Following the diet when eating out is hardest of all, the Princes say -- and they should know after spending several weeks on the road promoting the book. "Salt is our biggest enemy when we eat out. The next time I travel," Prince said, "I would insist on no salt."

But he admits you haven't much choice if your choice of dining is a greasy spoon. "You have to be a person of some means to eat out prudently."

Prince is ghosting five books now; his wife is teaching cooking and working on a sequel to the cookbook, temporarily titled "Diet for Life."

Price hasn't seen his docotr lately, or in the last several years, for that matter. "I think they are rediculous," he says."The cardiologist couldn't help me when I was at the point of death; he can't help me now."

Some adapted recipes from "The Dieter's Gourmet Cookbook" follow. RICE, PEAS AND MUSHROOM RISOTTO (4 to 5 servings) 1 cup white or brown rice, washed and drained 2 tablespoons corn oil 2 cloves garlic, minced 2 shallots, minced 1/2 pound fresh peas, shelled 1/2 pound fresh mushrooms, washed, dired, trimmed and sliced 2 teaspoons apple cider vinegar 1/4 teaspoon each dried oregano and marjoram leaves, cruched 1 cup water 1 cup chicken, veal, or beef stock* 3 dashed cayenne pepper Freshly chopped parsley *If serving fish use chicken stock.

Boil peas briskly in water to cover for 15 minutes or until tender, drain and set aside. Bring stock and water to boil. Add rice, and cook, partially covered, for 15 minutes. All liquid should be absorbed. Heat 3/4 tablespoon of oil in large nonstick skillet until hot. Add half of the mushrooms and saute until lightly browned, turning often (about 3 to 4 minutes). Transfer with slotted spoon to bowl. Add 3/4 tablespoon oil to skillet, and saute balance of mushrooms. Transfer to bowl.

Heat balance of oil in same skillet until hot. Add garlic and shallots and saute until lightly browned. Add vinegar and cook for 1 minute. Add cooked rice, peas, mushrooms, dried herbs and toss gently until heated through. Transfer to heated serving dish, and serve immediately, sprinkled with freshly chopped parsley.

Calories: about 180 per serving; sodium: with chicken stock 16 mgs, with veal stock 28 mgs, with beef stock 37 mgs; fat: 9; cholesterol: 0. SAUTEED CHICKEN LEGS WITH PIMENTOS (4 servings) 4 chicken legs with thighs, skinned, leg separated from thigh (total weight 2 pounds) 2 tablespoons corn oil 4 cloves garlic, finely minced 1 large onion, sliced 1 small green pepper, sliced into thin strips 1 teaspoon wine vinegar 1/4 cup chicken stock 1/2 teaspoons dried thyme leaves, crushed 1/2 teaspoons Dr. Bronner's seasoning* 1 large pimemto, no salt added, sliced 1/2 teaspoon ground ginger 4 dashes cayenne pepper *Available at health food shops.

Pat chicken dry with paper towels. Combine garlic, vinegar, 1/2 tablespoon oil, ginger and Dr. Bronner's seasoning in bowl large enough to accommodate all chicken. Add chicken, turning to coat. Cover, and refrigerate for at least 6 hours. Heat 1 tablespoon oil in large iron skillet until hot. Add chicken and satue to a golden brown on both sides, turning carefully with spatula. Skinned chicken must be handled gingerly. Remove from skillet and set aside. Heat balance of oil until hot. gAdd onion and green pepper to same skillet and saute until lightly browned. Add stock, thyme and cayenne and cook over medium heat, scraping to loosen browned particles (about 2 minutes). Return browned chicken parts and its juices to skillet, turning well. Bring small amount of liquid in skillet to simmering point. Reduce heat, cover and simmer for 20 minutes. Turn chicken parts and add pimemtos. Bring to simmering point, re-cover, and simmer for 20 minutes, stirring and turning once. Turn off heat and let sit, covered, for 30 minutes before serving.

At serving time, reheat over very low flame and serve piping hot.

Variation: The marinated chicken may be broiled, and -- voila! -- another dish.

Calories: 277, broiled 256; sodium: 91, broiled 80; fat: 15; cholesterol: 83. GREEN SALAD WITH FLAVORFUL DRESSING (4 servings) Salad: 1 small head romaine lettuce (about 3/4 pound) 1/4 pound chicory leaves 1/4 pound escarole leaves 1/2 bunch watercress, leaves only 1 onion thinly sliced 2 tablespoons freshly chopped basil, or freshly chopped parsley and dill 2 whole scallions, sliced 8 crisp radishes; sliced 1 endive, about 3 ounces (optional) Salad dressing: 1/3 cup apple cider vinegar 2/3 cup corn oil 2 teaspoons dried minced onions 2 cloves garlic, minced 2 shallots, minced 3/4 teaspoon combined dried marjoram and tarragon leaves, crushed, and dill and fennel seeds, crushed 4 dashes cayenne pepper 1 teaspoon freshly chopped chives

Remove each salad leaf and wash under cold running water. Drain and refrigerate in an uncovered bowl for 1 hour before serving. Pat each leaf dry with paper towels or use spinner. Tear leaves -- do not cut -- into bite-sized pieces.

Combined all salad ingredients except freshly chopped herbs and refrigerate for one hour. Prepare salad dressing by combining all ingredients in a jar and shaking well. Let stand one hour before serving. Shake again before serving. Allow 1 1/2 tablespoons dressing per serving. Pour over dried salad leaves to which freshly chopped herbs have been added. Toss well. Serve immediately.

Variation: Add 3 tablespoons tomato juice, no salt added, to salad dressing and shake well.

Note: This salad dressing and its variation will keep well in refrigerator for a week. Remove one hour before serving.

Calories: 37, with endive 38, with dressing 129, with tomato juice 130; sodium: 25, with endive 26, with dressing 28, with tomato juice 28; fat; 0, with endive 0, with dressing 14, with tomato juice 14; cholesterol: 0. PINEAPPLE CHIFFON PIE (8 to 10 servings) Pie crust: 1/2 cup rye flour 1 cup whole wheat flour 1/2 cup buckwheat flour 2 tablespoons date powder* 1/4 cup finely chopped walnuts 1/2 teaspoon ground cinnamon 1/2 cup corn oil About 3 tablespoons ice water Filling: about 1 cup fresh orange juice 1 can (12 ounces) crushed pineapple in its own juices, no sugar added 2 tablespoons honey (optional) 2 pre-measured envelopes unflavored gelatin, or 2 tablespoons gelatin 6 ounces dry curd cottage cheese, no salt added, less than 1/2 percent milkfat 1/2 cup low-fat plain yogurt 1 1/2 teaspoons vanilla extract 4 drops almond extract grated rind of 1/2 lemon 2 large egg whites 2 tablespoons walnuts, coarsely chopped *Available in health food shops.

Crust: In medium-sized bowl, combine flours, date powder, walnuts and cinnamon. Add oil, a little at a time, and blend with fork. Finally, dribble ice water, a tablespoon at a time, into mixture, blending with fork, using enough water to hold the mixture together. Turn into an 8-inch or 10-inch aluminum pie pan (not glass), and press even amounts against bottom and sides of pan. Bake in preheated 375-degree oven for 35 to 40 minutes until lightly browned. Cool pan on rack.

Note: If there are some white beans on hand, strew a cupful over unbaked shell so that dough won't swell while baking. When fully baked, remove beans and save for next baking.

Filling: Add enough orange juice to juice from can of pineapple to equal 1 1/2 cups liquid. Pour into saucepan together with honey. Bring to simmering point. Add gelatin, stirring to dissolve. Let cool to room temperature.

Combine cottage cheese, yogurt, and pneapple in blender. Add extracts and lemon rind, and puree on low speed for 1 minute. Pour into cooled gelatin mixture, and refrigerate until mixture begins to thicken.

Beat egg whites until firm, but not dry, peaks form. Drop 1/3 beaten egg whites into cheese mixture. Blend well with whisk. Pour balance of egg whites into mixture and fold gently using wooden spoon. Folding is done by making a down motion with the spoon, and then a scooping up and folding overt motion. In this way, all the egg white is incorporated into the mixture while air bubbles remain intact. Don't overfold. Cool in refrigerator until almost set. This will take about 45 minutes.

Turn cooled filling into baked pie shell. Sprinkle with coarsely chopped nuts and refrigerate. Allow 2 hours to set.

Variations: Alternate pie crust. Crunchy Rolled Oats Pie Shell 1 cup old-fashioned rolled oats 4 tablespoons honey 1/2 cup finely chopped walnuts 1/4 cup corn oil 2 tablespoons date powder

Toast oats by spreading them on cookie sheet and baking for 10 minutes. Transfer to plastic bag. Press all air out of bag. Then tie securely. Roll oats with rolling pin until they are broken up but not pulverized. In a bowl, combine oats with date powder and nuts. Add oil, a little at a time, mixing with spoon. Lift mass into center of an 8-inch ungreased pie pan and press to bottom and sides of pan. Dribble honey on crust and gently spread along sides and bottom of crust. Refrigerate for at least 1 hour before filling. The filling alone makes a superb dessert. Just pour into 8 dessert dishes and chill until firm. Sprinkle with chopped nuts and serve.

Calories: crust 123, filling 71, filling with honey 86; rolled oats crust 117; sodium: crust 0, filling 71; filling with honey 25, rolled oats crust 0; fat: crust 4, filling 0, filling with honey 0, rolled oats crust 7; cholesterol; 0.