A BIG PAN of freshly baked rice bread has just come out of the huge oven, and it looks -- and smells -- good enough to eat.

"The apes and monkeys and baby bears love it," says Moses Benson, head of the commissary at the National Zoo. His operation, where meals for the zoo's 2,500 animals are prepared, will be open for tours Sunday as part of a program on animal diets. Also included will be films and slides of animal diets and a workshop with a zoo nutritionist.

The rice bread, made of brown rice, raisins, baby cereal and honey, is one of the few items the commissary actually cooks, explains Benson.

"We also boil hundreds of eggs, dozens at a time, and grind them up here, shells and all," he adds, pointing to a machine makes a Cuisinart look like a toy. The eggs go into bird, monkey, iguana and other diets. Another humongous machine mixes chicken with a commercial cat food for the zoo's lions, tigers and other big cats.

Once a week, head keepers turn in grocery lists to Benson, who has an annual budget of about $500,000. The food is delivered to the commissary's loading dock and stored in the appropriate place.

"This is where we keep the fish, chicken, slab horsemeat . . . It's minus-twenty degrees," says Benson, opening the freezer with the flick of a switch and proving his words with a gust of Arctic air. Then there are special rooms for storing bananas -- one that's kept between 56 and 58 degrees for newly arrived bananas, and one that's a bit warmer for ripening bananas.

In a room that's about the temperature of a cool spring day, workers wearing coats and hats are sorting fruits and vegetables according to lists provided by the keepers and putting them into plastic bags. The labeled packages will be delivered to the animal houses at six the next morning. The keepers will cut up the vegetables and do the final meal preparation. There are carrots and apples and kale and sweet potatoes and honeydew melons.

"Those are for the fruit bats," explains Benson. "We usually use cantaloupes, but they're sky high right now."

Another room holds canned goods -- diet fruit cocktail for the birds, evaporated milk for the panda gruel, peanut butter for the parrots.

"They love peanut butter sandwiches," says Benson.

In addition to the stuff you might find at your neighborhood supermarket, there is more exotic fare: live mealworms for the marmosets; crickets, from a place called Jiminy Cricket Farm, for the reptiles; and oyster shells for the birds.

"Birds need calcium," explains research nutritionist Mary Allen, who sees to it that the zoo's residents eat balanced, healthful meals. One of her jobs is to devise formulas for such animals as zebras and marmosets and get commercial animal food companies to produce them.

Allen, who is basing her PhD thesis on an experiment involving feeding calcium to crickets and then feeding these mineral-enriched crickets to iguanas, started out studying domestic animal science.

"It seemed so production-oriented," she says. "I wasn't keen on getting chickens to lay more eggs."

So she went to the zoo, and now, in addition to providing the animals with healthy meals, she tries to keep them interested in their food.

How do you keep a monkey interested?

"We have something called the vegetable du jour," she explains. "One day it's broccoli, the next day it's cabbage, the next day it's green beans. We don't use a lot of bananas and other fruits. A high-fruit diet would be low in vitamins and minerals."

In Sunday's program, Allen will set up displays of the ingredients that go into animal diets, on tables in the Zoo's Mane Restaurant.

"I'll ask the participants to look at the food on display and pretend they had to feeda gorilla. What foods would they give it -- and why?"

And if you should spot a bottle of vitamin C on her desk, please don't think the tablets are for her. She's going "to grind them up for the iguanas." DIETING AT THE ZOO

Sunday's Zoo Diets program begins at noon in the Education Building, near the Connecticut Avenue entrance. Films on animal diets will be shown in the auditorium from noon to 4. Tours of the commissary will be available from 12:30 to 1:30, with shuttle service from the Panda Gift Shop and the Mane Restaurant. The diet workshops will take place in the Mane Restaurant at 1 and 2. Free tickets are required for the diet workshops only. They are available Friday through Sunday in the Education Building. Call 673-4717 to check on availability. No tickets are required for the films or the commissary tours.

Other coming family events at the zoo include:

ANIMALS IN STORY AND ART -- February 16 from noon to 4:20. Mask-making, puppet shows, storytelling, films.

A TROPICAL HEAT WAVE -- March 16 from noon to 4. Films about tropical forests, building a tropical mural, the game of Tropical Pursuit.