The giant pandas mated for the first time yesterday, but the encounter was so brief that the National Zoo's specialists aren't sure if it was a pairing that could produce a cub this summer or fall.

The reproductive attempt in the early afternoon only lasted about 15 seconds, assistant curator Lisa Stevens said last night, and staff at the Panda House had to study a tape of the male Tian Tian's performance to confirm that he and the female Mei Xiang got together.

A change in Tian Tian's vocalization was one indication of the mating, Stevens said.

The zoo's staff has been monitoring the pandas' every move in recent days because this is the one time this year that they could mate. Female pandas come into heat for only two or three days a year. A cub could be born three to six months later.

Until yesterday, the zoo said, Mei Xiang was showing nearly all the signs that she was ready to mate -- but she was still declining to do so. Tian Tian had been making periodic advances, including numerous unsuccessful attempts Thursday, and his efforts could continue today.

Tian Tian "gets an 'A' for technical merit and an 'A' for effort," said Stevens, who spent Thursday night in the Panda House after the zoo decided to start keeping the pandas together at night because of their increased sexual behavior.

The zoo has been conducting numerous tests to determine Mei Xiang's optimum fertility. Her estrogen levels, which started rising around March 25, actually dipped a bit yesterday. But there isn't enough reproductive history of this animal to know if she has reached her peak estrogen level for this mating season, or if the level will resume going up, Stevens said.

If the estrogen levels continue to drop, the zoo's hopes of producing a rare panda cub will have to be put on hold for another year.

"We're going to have to wait and see," Stevens said. "We're dealing with a young animal, and we don't have a lot of data on her."

The zoo's scientists, keepers and about 30 to 40 research volunteers are tracking the behavior of the pandas, with particular attention these days to sexual behavior. The pandas are scent marking and vocalizing more frequently, making come-hither chirps and bleats.

Zoo director Lucy H. Spelman said the public attention for the mating efforts will increase interest in the collaborative work the zoo is doing with China and other zoos to protect this endangered species. The pandas are at the zoo on a 10-year loan from China, and any cub eventually would be returned there.

"We are working with others to study them and protect them at zoos and in the wild," Spelman said. "The actual breeding gets people excited and interested in learning about this animal" and in a broader research program that is studying panda reproduction and habitat, she said.

The limited time-frame for mating is a key reason that giant pandas are so difficult to breed. About 1,000 giant pandas exist in the mountain forests of central China, and an additional 120 are in Chinese breeding facilities and zoos. About 20 giant pandas live in zoos outside China, including zoos in San Diego, Atlanta and, starting Monday, Memphis.

Mei Xiang means "beautiful fragrance" and Tian Tian means "more and more." They will turn 5 and 6, respectively, this summer.

Ling Ling, the zoo's previous female giant panda, had four pregnancies and produced five cubs, though none lived longer than a few days. But Ling Ling, who died in 1992 at age 23, was older when she began to breed, and was prone to urinary tract infections.

Mei Xiang is young and healthy.

The activity at the Panda House yesterday drew the usual spring crowd of visitors, which has numbered about 3,000 over the past couple of days.

Sharon Kaltwasser, her husband, Lynn, and their two sons, Stephen, 13, and Timothy, 10, who live in Lusby, spent the day at the zoo to celebrate Stephen's birthday. "We weren't aware she was in the mood," she said, as the family ate lunch at a table overlooking the panda yard. "But it sure was a good day to be here."

Randy pandas Mei Xiang, left, and Tian Tian romp at the National Zoo, which hopes the pair will reproduce. They mated for the first time yesterday, but officials don't know whether a cub is in the near future. Story on Page B3.