Panda watchers at the National Zoo said yesterday they have all but given up hope that Ling-Ling, the country's star female panda, is pregnant and announced plans to reopen the zoo's panda house to the public on Saturday.

Officials said they will continue to keep their eye on Ling-Ling through the end of the year. But she already has surpassed the farthest known reaches of panda gestation, they said, bringing to a close another tense and frustrating watch for the first healthy baby panda. This latest disappointment has opened the door to speculation that Ling-Ling may never be a mother.

"Right now, our best guess is that she probably is not pregnant," said Devra Kleiman, assistant director of research at the zoo. "Her biological clock is ticking. She's getting older and she's certainly a middle-aged or early old-aged panda now. We don't have a lot of time left."

Ling-Ling is believed to be 15 to 16 years old.

Kleiman said Ling-Ling may have gone through a false pregnancy after weeks of showing signs that she was expecting to give birth.

Ling-Ling and her male companion, Hsing-Hsing, mated four times in late June and early July. In mid-October, officials began to detect elevated hormone levels in the would-be mother and Ling-Ling started showing several signs of pregnancy.

Zoo officials closed the panda house to visitors Oct. 30 and Ling-Ling has been under constant surveillance ever since. Daily samplings of her urine have been shipped to the Bronx Zoo in New York for hormone analysis.

Ling-Ling has been carrying branches of bamboo to a corner of her cage as if preparing a nest, zoo officials said. Watchers also have seen her cradling apples and sitting hunched over for long periods. And for the last six weeks, Kleiman said, the panda's eating habits have been "very erratic."

But even allowing one or two months' leeway in the unpredictable gestation period, Ling-Ling has passed her 168th day since mating. Pandas normally don't give birth after about 165 days, Kleiman said.

Kleiman said that officials have not given up on the panda pair and that another mating watch probably will commence early next spring.