Ling-Ling, the National Zoo's female giant panda whose long history of romantic trials and gynecological tribulations have helped make her one of the foremost four-footed celebrities in the city and the nation, may once again be pregnant.

Zoo officials said they have seen signs that warrant closing the panda house, starting tomorrow, to provide privacy for the potential mother-to-be.

However, they cautioned that they are not certain that Ling-Ling is pregnant. If she is, it would be for the third time in her 15 years of life. Her first cub died three hours after birth; the second was stillborn.

"We're hopeful," said Dr. Robert Hoage, the zoo's chief of public affairs. "I think the word is hopeful."

If the hope is well founded, Ling-Ling could deliver as soon as a few days from now, or as late as mid-December, Hoage said.

Female giant pandas often weigh about 250 pounds. Their cubs, however, weigh only about four ounces at birth. As a result of this vast disparity, pandas show no obvious outward physical signs of pregnancy.

Experts trying to anticipate an impending birth look instead for changes in behavior and in body chemistry.

One sign prompting an upsurge in optimism among zoo officials is a decline over the past week in Ling-Ling's appetite.

She no longer shows her usual enthusiasm for her bamboo, carrots and apples, or her vitamin, honey and rice gruel.

When the panda house closes, volunteers will join zoo staff members in looking for other behavioral changes, such as restlessness and anticipatory nest building.

The other sign that has aroused hopes of a 50 percent increase in the District's panda population is based on body chemistry. Ling-Ling's urine has been tested for months; the most recent test for which results are available finally showed the hormone levels associated with pregnancy. Results of another test are being awaited.

The pregnancy watch follows mating on June 30 and July 1 and 2 between Ling-Ling and her companion, Hsing-Hsing, the male member of Washington's giant panda pair.

Over the more than 13 years since the pandas came here as gifts from China, their romantic relationship has been the subject of vast popular interest and a matter of considerable physical difficulty.

In this year's mating, science helped remedy the defects of nature. After Ling-Ling failed to start her mating cycle, which had for the last three years occurred in mid-March, she received hormone injections.

Now, with Ling-Ling in panda middle age, zoo officials say the evidence indicates the "chance of a pregnancy."