The spring fling at the Panda House is over.

That's the conclusion the National Zoo's giant panda specialists have reached after seeing a marked change in the behavior of their prized animals -- and reviewing the latest lab results.

Mei Xiang, the female, is continuing to try to attract the attention of the male, Tian Tian. But hormone tests showed that her estrogen levels have dropped for two days in a row and that the time of peak fertility has come, and gone.

And Tien Tien, who on Thursday and Friday often couldn't keep his paws off Mei Xiang, gave her the cold shoulder yesterday.

"Her hormones have dropped, and he's lost interest," said assistant curator Lisa Stevens. "His behavior is indicating that it's over."

This limited time-frame for mating is one reason that giant pandas are so difficult to breed. The population of the endangered species has dwindled to about 1,000 in the wild in China and 140 or so more in zoos and breeding facilities.

There is a slight chance that the pair's brief, 15-second close encounter on Friday, when they mated for the first time since coming to the zoo in December 2000, could produce a cub this summer or fall.

Stevens said the zoo will start checking in about 40 days to see whether Mei Xiang's progesterone levels start going up. If they do, the zoo would then start a pregnancy watch. Giant pandas give birth about 90 to 184 days after mating, although females, including Mei Xiang's predecessor at the zoo, Ling Ling, have sometimes experienced pseudo-pregnancies.

Ling Ling and Hsing Hsing, gifts from China in 1972, were at the zoo 11 years before they finally mated, and by then the female was middle-aged. Mei Xiang and Tian Tian, who turn 5 and 6, respectively, this summer, are still young, and the fact that they have mated so soon is a good sign.

"I'm very pleased with everything," Stevens said. She remained hopeful that the brief mating Friday might lead to a cub, but she would have preferred it if they had mated several times.

"If we don't get a pregnancy this year, it's okay if it's next year," she said.

Stevens said zoo staff is looking forward to continued studies of the pandas' reproductive behavior so they can get a better breeding history of both animals.

The giant pandas are on loan from China through 2010, at a cost of $1 million a year toward panda conservation efforts in that country. Any surviving cub eventually would be returned there.

Keeper Brenda Morgan gives Mei Xiang a snack after a morning test. Tian Tian lost interest after his counterpart's hormone levels dropped, zoo officials said.