When the new panda cub died at the National Zoo a few weeks ago, everyone there was very sad. They were sorrowful because the baby animal was gone. They were also discouraged because the death was a setback for the worldwide panda breeding effort that has been going on for many years now.

Even though this baby died, researchers at the zoo hope that nature will take its course, and Ling-Ling and Hsing-Hsing will have another chance at parenthood next year.

The giant panda is an endangered species. There are only about 1,000 left in the world. About 90 live in zoos in China, and a few in other zoos around the world. China has had the greatest success with breeding the animals; 15 babies have been raised at the zoo in Beijing.

But since so few of the animals still live in the wild, and the area where they live -- a special high, damp, bamboo forest -- is shrinking, the wild giant pandas are in danger of dying out.

Once a particular kind of animal, or species, is gone, it is gone for good. During this century, zoologists have placed the names of many, many animals on the endangered species list. As we human beings take up more and more space for our towns and cities and farms, animals lose their habitats -- the special places in which they thrive. Other problems like pollution, unlimited hunting and lack of certain kinds of food also have threatened some animals.

When you go to the zoo, you have a fun day with your family and friends. You marvel at the giraffes, giggle at the plump little pygmy hippos, and are amazed by the antics of apes. You eat peanuts or ice cream, and stand in line to see the pandas. Zoos are great places to visit.

But zoos are much more than just amusement parks that happen to have a collection of exotic animals. They also contain scientific laboratories and breeding facilities where experts are hard at work trying to figure out ways to protect the earth's endangered animals.

One important way of doing this is to make sure healthy babies continue to be born. Some species will benefit from artificial breeding. These methods include producing babies through in vitro fertilization -- mixing egg cells from a female and sperm cells from a male in a dish in a lab, and then putting the fertilized egg into a female's body to grow.

Another new technique is to freeze embryos -- tiny, unborn babies -- from endangered animals. The frozen embryos can be saved, and then placed in a healthy female where they grow until birth.

Dr. David Wildt heads the National Zoo's Reproductive Physiology program. Part of the zoo's animal health department, the program was founded in 1983. "Our job is to learn more about how animals reproduce," he says. "You have to do that before you can artificially breed them."

Dr. Wildt and researcher Karen Goodrowe have been studying a familiar, non-endangered animal, the domestic cat -- the kind you or your neighbor might have. This year, they have been successful at breeding three litters -- a total of nine kittens -- by in vitro fertilization. They hope that their success will eventually lead to artificial breeding for some species of endangered cats.

"No one had ever done in vitro fertilization with any carnivore before," Dr. Wildt says. "It's a major breakthrough." Carnivores are a large group of meat-eating animal families including cats, dogs, raccoons and bears. "There are some endangered South American and Asian cats that could benefit," Dr. Wildt says. "One is the Pallas cat, the wild ancestor of the Persian cat that is a very endangered Asian species. We're also working with a small colony of leopard cats, and we have plans to work with tigers and cheetahs, too."

The zoo is building a new hospital facility to house some of the research programs it has under way. "We have a training program to do research in zoos," says Dr. Wildt. Karen Goodrowe, who worked on the cat project, is its first graduate. "We also work in the field in Africa, doing studies on elephants, cheetahs, lions and so on," she says. "We're studying the biology of the animals."

Eventually, the scientists hope to be able to bring back reproductive cells from the wild animals, but without having to take the animals out of their natural habitat. "We could use the material for breeding with animals already in zoos, and avoid taking any more animals out of the wild."

Next time you go to the zoo, remember that the people there are doing much more than entertaining you. They're hard at work to ensure the survival of as many animals as possible.

You may be wondering what happened to those nine kittens that were born through in vitro fertilization. Well, they're doing fine. Through the Friends of the National Zoo, all nine were adopted by good homes. ::

Tips for Parents

FONZ -- Friends of the National Zoo -- provides a wide range of activities for kids, including a summer safari day camp, a variety of classes, and a junior club for ages 12 to 16 in which youngsters earn a "Zoo patch" by doing an in-depth observation of some part of the zoo, and take part in puppet shows and other activities. A family membership in FONZ is $35. For information, write FONZ Membership Dept., c/o The National Zoo, Washington, D.C. 20008; phone 673-4950. The National Zoo education office also offers a variety of opportunities for families, including the hands-on activity areas ZOOlab, BIRDlab and HERPlab. For information, call 673-4800.

Catherine O'Neill is a free-lance children's writer in Baltimore.