First the good news: After years of trying to breed a giant panda cub, veterinarians at the National Zoo believe they understand the reproductive mystery behind female Ling-Ling's occasional pseudopregnancies.

They still can't tell for sure when she's pregnant. But they can tell, earlier than in the past, when she's not.

The bad news is that the 21-year-old panda didn't get pregnant this year, but only acted like she was. Officials called off the celebrated "panda watch" July 11, with considerable certainty that Ling-Ling had experienced another false pregnancy.

The gestation of panda cubs may last 78 to 170 days because implantation of fertilized panda eggs in the uterine wall may be delayed for weeks after conception. Ling-Ling, who may have mated last winter, could have given birth as late as August. But animal researchers at the zoo have learned to monitor hormone levels in her urine so precisely that they knew when the pseudopregnancy ended.

The National Zoo has been trying to breed a giant panda cub since 1976. Ling-Ling and her mate, Hsing-Hsing, gifts from China, had a few problems figuring out what to do at first. Since 1983, Ling-Ling has given birth to five cubs, the longest-lived dying of an infection after nearly four days.

The panda mother also has had four false pregnancies, including the one this year, during which she showed hormonal signs of being pregnant. After monitoring Ling-Ling for several years, veterinarians have concluded that she and other female pandas who ovulate during their brief mating cycle will produce protective, pregnancy-like hormones even if they are not pregnant.

If ovulation occurs, the zoo says, there will either be a cub -- assuming conception takes place -- or a pseudopregnancy.

"It's not possible at this time to tell the difference between a true pregnancy and a pseudopregnancy," said Steven Monfort, an endocrinologist and research veterinarian at the zoo. "But when the hormone levels rise and stay elevated for four to eight weeks and then drop to zero, and there is no baby, then we know it was a pseudopregnancy."

This is a particularly helpful thing to know at the National Zoo, where a lot of effort goes into preparing for the possible birth of pandas, an endangered species. National Geographic moves in camera equipment, and scores of volunteer panda watchers are pressed into service.

Now, Monfort said, the zoo will have a better idea of when to keep everyone on alert -- and when to send them home.

Will all this matter now that Ling-Ling, the oldest panda in captivity, is getting on in years?

Ling-Ling obviously is still experiencing normal ovulations, Monfort said. And no matter what happens in the future, the data gained from studying her reproductive history will help other panda breeders.