Look how they're restocking zoos now, Noah

By itself, the birth of a rare animal in captivity is uncommon enough. But when two African bongo antelopes rolled out onto some straw matting at the Cincinnati Zoo recently, there was an unusual twist to the saga.

The wobbly white-striped newborns were delivered one week apart from two female antelopes - while their real mother lay halfway across the country in a stall at the Los Angeles Zoo.

The bongos were the product of an emerging technology - embryo transfer - that is becoming a powerful new weapon for the zoo keeper in the fight to save endangered species. The technique is widely used in reproducing cattle and some other common mammals. But among exotic species it has remained largely in the teething stage. Within the past month, however, two unusual births have boosted its potential in eventually helping replenish populations of a wide variety of threatened species:

* First, the Cincinnati experiment. Last year, after a bongo conceived at the Los Angeles Zoo, scientists flushed several embryos from her using a rubber tube and sterile solution. Then, toting the tiny cell clusters in glass vials strapped to their arms, they spirited them back to Ohio by jet and lodged them in five ''surrogate'' mothers: four common eland antelopes and a female bongo. In early June, the auburn newborns were delivered - the first successful births of exotic animals from a cross-country transfer.

* The other event was the first successful transfer between different equine species - a zebra and a horse - at a farm outside Louisville, Ky. About a year ago veterinarians extracted a 10-day-old embryo from a pregnant Grant's zebra housed at the Louisville Zoo and implanted it in the womb of a quarter horse. The horse gave birth to the striped newborn late last month.

Zebras aren't endangered species, of course. But Louisville Zoo officials expect the technique will be used on rare equines, such as the wild Asian ass, Burchell's zebra, and Przewalski's horse. It may also help conserve species with similar reproduction systems, including rhinos and elephants.

''We are opening up a whole new world that has yet to be pioneered,'' says William Foster, assistant director of the Louisville Zoo. ''It's not going to solve all our endangered-species problems. But it is giving us a new tool.''

The allure of embryo transfer lies in its potential to rapidly replenish populations of animals without some of the problems of traditional breeding. By allowing a host mother to carry young through pregnancy, the donor is freed up to produce more offspring. The bongo, for instance, normally carries its young for 278 days, which would mean one newborn a year. But by enlisting the help of other mothers, an entire family can be reproduced quickly.

The technique may also allow offspring to be generated without having to transport animals from the wilds to controlled mating locations - a costly and often risky practice. Then, too, many animals simply don't breed well in captivity. The Cincinnati group, headed by Dr. Betsy Dresser, is now planning to freeze a bongo embryo and then transplant it - a step that would make it easier to transport species from around the world.

Tampering with nature's birthing process is not a new idea. Embryos were first ''harvested'' in rabbits in 1890. Since then the technique has been applied to dozens of mammals, most often cattle. Breeders routinely use it to increase the number of offspring produced by prize cows (e.g., ones with superior milk-producing traits).

More recently, scientists have begun splitting embryos - in effect, ''cloning'' twins or sets of twins - to further increase a top pedigree's brood. This has been done with cattle, and the technique is being worked on in horses. It is the potential of applying these techniques to human beings, particularly cloning, that raises complex ethical and legal questions. The depth of concern involved is reflected in a flap now going on in Australia. One scientist in a team that has pioneered the storage of frozen human embryos recently resigned, partly because of moral concerns over the potential impact of the work. A human embryo transfer has been performed in California. Scientists believe cloning is technically possible.

With endangered animals, however, the practice has generally been welcomed. Only a few exotic species have been delivered so far: one at New York's Bronx Zoo (a wild ox from a dairy cow), one at Utah State University (two Sardinian sheep from a domestic sheep), and the ones at Louisville and Cincinnati. How widely the technique may be applied remains uncertain. Far less is known about transfers in exotic than domestic species, and the success rate so far has not been dramatic.

Still, zoo keepers see it as one more tool to go along with stronger wildlife protection, better natural breeding, and other practices that may eventually help save some species from extinction. ''This technology is going to be very important to the future of a lot of wildlife,'' says Dr. Dresser.