"Half the family pickets McDonald's. The other half eats there," says Riki Singer, carnivorous mother of one burger worshipper and two vegetarians. Not only that, but half of the refrigerator in the Singers' Alexandria kitchen is sectioned off for the vegetarian contingent and half of the time the dinner conversation turns into an animal-rights debate.

According to the most recent statistics from the Roper Organization, there are approximately 7 million Americans who are vegetarians. Almost 38 million people are "careful" about how much meat they eat, according to Roper, as more and more Americans are looking at the health merits of a meatless or meat-moderate diet.

But, while bean sprouts are no longer the domain of '60s flower children, there is still a coterie of teen vegetarians, 1985-style. In this era, however, it may be harder to do vegetarianism right. Hungry and growing bodies tempted by the proliferation of fast-food fries, shopping-mall snacks and school vending machines are perhaps particularly vulnerable in making a nutritious switch.

Some teen-agers "think vegetarianism is just giving up meat," says Margie Ginsberg, a local nutritionist. Consumption of candy bars, chips and cookies may be increased to flesh out the diet, meal choices are unbalanced, or sources of animal foods are not fully understood so that vegetable soup made with chicken stock, for instance, may be inadvertently included.

The reasons a teen gives up meat? It is often ethically motivated, according to a spokesman from the Vegetarian Information Service in Bethesda."Most of the vegetarians in the school are truly repelled by the idea of eating meat," reads an article in the T.C. Williams High School newspaper about the small but growing group of vegetarians in the school. "They also oppose the killing of animals, and they seek better health through diet," the article continues.

"We used to be very materialistic," says Singer's 13-year-old son, John Rosenberg, of himself and his brother Bill, both ex-fast-food junkies. "We spent $5 a day on video games." Now John, whose mother pays him $5 a night for cooking dinner for both the meat-eating and nonmeat-eating family members, and 15-year-old Bill, who makes money mowing lawns, donate their earnings to world hunger and animal-rights movements.

"It's vegetarianism the greatest way to revolt," says Bill Rosenberg, adding that he feels his eating habits are a much more constructive rebellion than drugs, alcohol or cigarettes.

In her "most romantic vision" of her son's vegetarianism, says Penelope Lemov, mother of 17-year-old Doug, a senior at Walt Whitman High School, "he saw a world of chaos . . . It was a way of imposing some peace and order." Her other vision, says Lemov, was his search for a sense of identity, a distinguishing feature. Doug Lemov, who was a vegetarian for three years, has recently gone back to eating meat.

It can also be one of a parade of teen-age phases. Pat Sappe, cafeteria manager at Rockville's Woodward High School, says that there may be 20 to 30 consistent vegetarians at the school, but there are plenty of "the on-and-off" types -- the kind who will shun meat entrees one month and grab for hot dogs the next, says Sappe.

Even the vegetarians go through stages. Tanya Klish, a 15-year-old quasi-vegetarian who attends Bethesda-Chevy Chase High School and doesn't eat meat because she doesn't like the taste, said she used "to love" Kraft Macaroni & Cheese, then it was Spaghetti-O's and now it's tuna. And in his eight months off meat, Bill Rosenberg has gone from being a vegetarian to a vegan (a vegetarian who eats no dairy products) to a fruitarian. One day recently Rosenberg ate 12 apples for lunch.

In the worst-case scenario, vegetarianism is cropping up among female teens with anorexia nervosa. There's "no epidemic on the scene" of anorectic vegetarians, says Dr. Tomas Silber, director of adolescent medicine at Children's Hospital, but it's "not unusual" that teens "obsessed with food" turn to vegetarianism.

"It gives them the anorectics an excuse not to eat with the family, to make more of an issue out of food," says Mary Dickie, senior clinical dietitian at Children's Hospital, who worked with anorectic teens for four years.

If executed properly, a vegetarian plan may be a healthier alternative to a teen-eating regimen loaded with junk food. Vegetarians who eat fruits and vegetables will often get vitamins and minerals lacking in other teens' diets, says Chevy Chase nutritionist Ann Litt.

Silber said he's "not worried" about vegetarian teen-agers, as long as they balance their meals with dairy products. It's not a good idea for a teen to become a vegan, according to Silber, because it could lead to calcium and protein deficiencies.

Jennifer DiNucci, vegetarian mother of 15-year-old Derrick, a strapping weight lifter who has been a vegetarian all his life, says the family doctor "doesn't believe he's vegetarian." She says the doctor has asked, "Are you sure you're not sticking meat in his meals?"

A good vegetarian regime can often lay a good groundwork. Lemov says that although her son Doug is no longer a vegetarian, he is more aware of food and has adopted a healthier eating style.

In fact, some vegetarian teen-agers have influenced their households so that the whole family cuts down on meat. Lemov says she currently eats less meat than her son does, and the Rosenberg brothers sometimes pack their mother a vegetarian lunch to take to work.

But aside from not eating meat, some local teen vegetarians want to know more. "Sometimes I feel like I'm not getting my requirement where the meat was," says Tanya Klish. "I want to learn how to be a better vegetarian," Klish says. Derrick DiNucci, who says he's not into junk food, wants to know more about nutrition. "I want to know what vitamins and minerals are in the foods I eat," DiNucci says.

At least a few local vegetarian teens have gained weight since they made the switch. Gail Shusterman, an 11th grader at Woodward High, who says she often buys pastries and bread for lunch at Sutton Place Gourmet, says it's "hard to lose weight on a vegetarian diet." Vegetarian foods high in protein, such as nuts, says Shusterman, are also high in calories.

Some carnivorous parents, worried about their vegetarian teen-agers, have taken them for professional advice. Tamar Grossman and Rachel Durham, two Woodward High students, said their parents took them to their doctors, and Lemov said she took her son to a nutritionist because her husband insisted.

Then there's the parental or sibling reaction to a vegetarian in the house. Meat-eating Mark Rosenberg, 11-year-old brother of Bill and John, says he feels "interrogated" by his brother Bill, and often squabbles with him, spraying air freshener in the kitchen when he has made lentil burgers, or arguing with him about the merits of a vegetarian diet for Mitzi, the family dog.

Lemov, who said she and her husband "felt strongly" that if her son wanted to express himself through vegetarianism that they should be supportive, said her husband finally got Doug to eat shrimp, scallops and mussels by convincing him that they were "literally the vegetables of the sea."

And how do vegetarians deal with peer approval -- or disapproval? "Some people think it's weird," says Cheryl Petty, a senior at Woodward High. Some kids at school have taunted his brother, according to John Rosenberg, saying things such as 'Bill, I just ate roast beef, it was so good.'"

But sometimes, it's catching. John Rosenberg has convinced his girlfriend to become a vegetarian, he says, and gave her a copy of "Laurel's Kitchen" to read.

Grossman says she has convinced seven friends to become vegetarians, although sometimes mothers "get mad" at her if their son or daughter isn't really committed to it.

One of the hardest situations, says Cheryl Petty, is going to dinner at the house of a friend whose parents may not know that she is a vegetarian. Petty says she sometimes will take a small portion of the meat entree and just "mix it up" in her plate, making it appear as though some has been eaten.

Or sometimes parents will think that because you're a vegetarian, all you eat is steamed vegetables, and they'll serve you a huge plate of broccoli, says Gail Shusterman.

One way that many families deal with a vegetarian in the house is to make side courses for meat eaters into main courses for vegetarians. Margo Klish says she often makes a vegetable casserole as a side dish for the rest of the family and Tanya will eat it for a main course.

Here are some recipes that the whole family -- carnivores included -- can share. VEGETABLES, MACARONI & CHEESE (4 to 6 servings)

2 scallions, sliced

1/2 cup margarine or butter

3 tablespoons whole wheat flour

2 cups milk

1 cup low-fat cottage cheese

1/4 cup chopped parsley

Salt to taste

1/2 teaspoon pepper

1/4 teaspoon thyme

Dash garlic

Dash nutmeg

4 cups cooked macaroni noodles

1/4 cup toasted wheat germ

3 cups cut-up cooked vegetables

1 cup grated cheddar or swiss cheese

1 cup whole-grain bread crumbs

Saute' scallions in margarine until soft. Blend in flour and cook over medium heat several minutes, stirring constantly. Slowly add the milk, continuing to stir. Add the cottage cheese and seasonings. Cook until sauce thickens. Combine noodles, wheat germ, vegetables and sauce and pour into a greased 2 1/2-quart baking dish. Top with cheese and bread crumbs. Bake at 350 degrees until piping hot and bubbling, about 15 to 20 minutes. From "Laurel's Kitchen," by Laurel Robertson, Carol Flinders and Bronwen Godfrey WHOLE WHEAT PEANUT BUTTER PANCAKES (Makes about 12)

1 cup whole wheat flour

1/4 cup nonfat dry milk powder

1/2 teaspoon salt

2 teaspoons baking powder

2 eggs, beaten

1/4 cup honey

1/2 teaspoon vanilla

1/2 cup peanut butter

3/4 cup water or milk

Stir together the flour, milk powder, salt and baking powder. In a separate bowl, beat the eggs, honey, vanilla and peanut butter. Then mix in the water or milk. Add the liquid mixture to the dry and stir well until ingredients are smooth. Bake on a hot skillet, flipping once.

Note: To make peanut butter waffles, increase water or milk to 1 1/4 cups. From "Recipes for a Small Planet," by Ellen Buchman Ewald LENTIL-WALNUT BURGERS (4 to 6 servings)

3/4 cup dry lentils

1 1/2 cups water

2 teaspoons cider vinegar

1 tablespoon butter

1 cup finely minced onion

1 to 2 cloves garlic, crushed

10 large mushrooms, minced

1/2 cup finely minced walnuts

1 small stalk celery, minced

1 teaspoon salt

Generous grinding of black pepper

1/2 teaspoon dry mustard

1 tablespoon dry sherry

1/2 cup wheat germ

Cheese and basil for serving (optional)

Bring lentils and water to a boil in a saucepan. Lower the heat and simmer, partly covered, 30 minutes, or until lentils are soft and liquid is gone. Place in a large bowl. Add vinegar, and mash.

Melt butter in a saucepan. Add onion, garlic, mushrooms, walnuts and celery and saute' together over medium-low heat 10 to 15 minutes, or until all is tender. Add salt, pepper, mustard, sherry and wheat germ to the mashed lentils and mix well.

Chill for about 1 hour before forming patties. Form 4-inch burgers and fry in butter until brown, or broil about 8 minutes on each side. If desired, melt cheese on top and sprinkle with basil. From "The Moosewood Cookbook," by Mollie Katzen BUILD-A-BURRITO

Refried beans

Chopped tomatoes

Chopped green peppers

Chopped onions

Salsa

Grated cheddar or monterey jack cheese

Sour cream

Mashed avocados

Flour or corn tortillas

Place beans and other condiments in bowls. Warm tortillas in a 325-degree oven on a cookie sheet. Be careful, this only takes a few minutes, any longer and the tortillas will be crispy. Build a burrito by placing any or all of the condiments on top of tortillas and roll. ALL-PURPOSE RATATOUILLE

Olive oil

Chopped garlic

Chopped onion

Chopped green peppers

Cubed zucchini

Cubed yellow squash

Cubed eggplant

Chopped tomatoes

Basil, thyme and oregano

Salt and pepper

This dish can be adapted to any quantities or combinations of vegetables you prefer. Heat olive oil in a skillet. Add garlic, onions and green peppers and saute' until onions are translucent. Add any other vegetables and the herbs and cook until tender. Serve over pasta topped with parmesan cheese, in a pita sandwich or with a green salad.