Ling-Ling's newborn giant panda cub remained in good health and locked in its mother's embrace yesterday even as red-faced officials at the National Zoo announced they had discovered the body of a second cub, apparently stillborn, that had gone undetected in all the excitement of Tuesday's successful birth.

"Ling-Ling had twins," said Benjamin Beck, the zoo's general curator. Watching the birth on a closed-circuit television monitor, "we simply did not see and could not see when it was born, but we think it was just before the live birth," he said.

The dead cub was spotted Tuesday afternoon by a keeper who was walking past Ling-Ling's enclosure in the Panda House after feeding her next-door mate, Hsing-Hsing, according to zoo officials. Late yesterday morning, when the giant mother panda left her nest and moved into her den, taking the live cub with her, keepers retrieved the other cub's body and took it to the zoo's pathology lab.

Robert Hoage, a spokesman for the zoo, later reported that the dead cub appears to have been a male. Zoo scientists have not yet determined the cause of death.

It is not unusual for pandas to give birth to twins, but there is no known case outside of China of both surviving.

In other circumstances, Hoage said, the stillborn birth of a twin cub would raise fears about the health of the surviving panda. "There is some concern," he acknowledged, "but the fact that the infant appears to be very strong and is vocalizing so loudly reassures us."

Zoo authorities and some members of the news media got a good look at the newborn yesterday morning when Ling-Ling got up briefly from her nest, revealing a strong cub that struggled up on all fours, moved a little and whined loudly for its mom.

Having repaired to her den, Ling-Ling and her 4-ounce, 5-inch-long cub are now out of camera range and no longer visible on the zoo's television monitor. But Hoage said a keeper checks on them periodically, and all seems normal.

The live cub, which arrived at 3:33 a.m. Tuesday, is the third and longest lived offspring born to the celebrated panda couple, a gift from the Chinese government in 1972. Its continued survival would make it the first successfully bred panda in the United States, and zoo officials said yesterday they hope eventually to mate it with another panda and try to increase what is now considered an endangered species.

To that purpose, according to Devra Kleiman, the zoo's assistant director for research, "It's definitely better if this cub is a female -- there are already more males than females in captivity."

Kleiman said it may be several weeks before zoo veterinarians will know the sex of Ling-Ling's surviving cub. "We can tell the sex at the point when we can first go in and handle the cub, and we wouldn't do that if it might jeopardize the mother-infant bond," she said.

But regardless of whether the new cub helps perpetuate its species, animal conservationists here and around the world have already learned much from the soap opera love life of its parents.

Kleiman, ticking off the scientific gains at a news conference yesterday, said the National Zoo now knows more about the panda's behavioral changes preceding and accompanying heat and about the pregnancy cycle itself. Also, having done copious hormonal analyses during Ling-Ling's real and what turned out to be "pseudo" pregnancies, "I think we can tell the difference," she said.

At 18 years, "Ling," as she is known affectionately by her keepers, is one of the longest-lived pandas in captivity. Scientists believe the normal life span for pandas, members of the bear family, is about 20 years.

The 245-pound mother's tiny cub will develop slowly over the next few months and probably won't be on public view until November at the earliest. Right now, the cub is a "pinkish white" and won't start to show a panda's characteristic black markings for a week or 10 days. Its eyes are closed and won't begin to open for another month or so.

"The Giant Panda," a book published in China and written by Zhu Jing and Li Yangwen, says that at two months, the cub will weigh 6 to 8 1/2 pounds and start to crawl a little. At 5 months, the cub will weigh more than 22 pounds, be able to walk about freely and begin to wean itself gradually from its mother.

A year-old panda, Kleiman said, is still comparatively very small, weighing less than 90 pounds.

Although Ling-Ling's forearm and hunched-over position have obscured the view, Lyndsay Phillips, the zoo's associate veterinarian, said he and others are confident the cub is nursing from its mother. If it were not taking nourishment, its vocalizations would be weaker, he said.