The director of the National Zoo said this week that Ling-Ling, its giant female panda, is sending mixed signals about being pregnant but that her keepers are still hopeful she will give birth before Dec. 17, when the 24-hour "Panda Watch" is scheduled to end.

"We are running out of time, and I was beginning to get discouraged," said Dr. Michael Robinson, the zoo's director. "But over the weekend she started indulging in all kinds of prebirth activities, so we're encouraged again."

Robinson said the 15-year-old Ling-Ling has begun moving nesting material into her den, is cradling apples against her stomach as if nursing a cub and is licking her genital area, all signs of a probable pregnancy.

However, he said, a recent hormone test "was less consistent with her behavior" and has her veterinarians somewhat puzzled.

The recorded gestation period for pandas has varied from 97 days to 168 days, zoo officials say, and Ling-Ling is about 158 days into her gestation, if she is pregnant.

The round-the-clock volunteer "Panda Watch" will end Dec. 17, although Robinson said zoo officials "can conceive, if that's the right verb to use, of her still giving birth after that date."

Robinson said the zoo has data only from Ling-Ling's two earlier pregnancies in 1983 and 1984 and from a "false alarm" in 1982.

One way or the other, Robinson added, the information gathered this time will be very useful in studying panda behavior.

Ling-Ling's first cub was born in July 1983 but died a few hours later from a respiratory ailment. A second cub was stillborn in August 1984.