A victory the size of a stick of butter was won today in the fight to save one of the world's most imperiled species: the giant panda.

As nervous researchers from the San Diego Zoo watched over closed circuit television, the female panda Bai Yun gave birth to a tiny cub shortly before noon.

If the infant survives--and initial indications are good--it will become the first panda born in a zoo in the United States to survive beyond a few days. The San Diego Zoo has the only panda pair in the United States, although the Atlanta zoo is scheduled to receive a pair this fall from the bear's native China.

The sire, Shi Shi, was reportedly nonplussed, but San Diego researchers hailed the birth as a possible breakthrough in unlocking the mysteries of the panda's reproduction system.

The two pandas came to the San Diego Zoo in 1996 as part of a 12-year reproductive research loan from China. Plans are to keep mother and infant off public display until the nursing and dependency period ends in December.

As the numbers of pandas dwindle in China because of poaching and the loss of habitat, researchers in the United States and China are scrambling to prevent the extinction of the charismatic species with the black-and-white fur.

"It's been a culmination of a lot of work," Don Lindburg, the zoo's panda team leader, said shortly after the birth.

Under the loan agreement between Chinese officials and the San Diego Zoo, the as-yet-unnamed cub is owned by China and will be shipped to Wolong, China, after three years. The zoo will also pay the Chinese up to $600,000, along with the annual $1 million contribution to Chinese efforts to preserve the panda's bamboo forest habitat.

Although pandas have been on display at zoos in Europe, America and Mexico since the 1920s, reproduction in captivity has proven spotty, for reasons that researchers have been unable to fathom. Pandas at the Mexico City zoo have reproduced successfully but not since 1990.

So eager are Chinese researchers to boost the panda population that they have begun giving the impotence drug Viagra to male pandas. Triplets were born in captivity a week ago, but one has since died.

A panda pair at the National Zoo in Washington, D.C., were never able to produce a cub that lived beyond a few days. Ling-Ling, the female, has since died, and Hsing-Hsing, the male, is terminally ill.

At San Diego, researchers are monitoring mother and baby and are prepared to intervene if necessary. Panda mothers have been known to smother their infants.

"A lot can go wrong in the first days," said Lindburg, "but so far, we're very confident. Bai Yun has bonded very well with the infant."

Bai Yun, which means White Cloud, was artificially inseminated three times in April with semen from Shi Shi, whose name means Rock. Bai Yun, who is 8 years old, had been willing, even eager, to mate with Shi Shi, who is 20-plus, but he had shown no interest.

The birth should provide researchers with information about how and when to inseminate females. Also they have learned more about how to support a pregnancy with appropriate nutrition and comfortable environment.

CAPTION: Bai Yun, a female giant panda, gave birth Saturday to a cub in San Diego after being artificially inseminated in April.