Frustrated but undaunted in their efforts to impregnate Ling-Ling the giant panda, National Zoo officials undertook an elaborate effort over the weekend to artificially inseminate the animal.

Frozen panda sperm was flown in from the London Zoo on Friday to supplement both fresh and year-old frozen samples taken from Hsing-Hsing, the National Zoo's 11-year-old male panda, according to zoo officials.

"We are covering all the bases," zoo director Theodore H. Reed said yesterday. "We are using everything we possibly can to make sure that gal gets pregnant."

Both Ling-Ling and Hsing-Hsing were operated on by a team of about a dozen medical and zoological specialists at the zoo hospital, Reed said. Separate insertions of fresh and frozen sperm from the two different male pandas -- Hsing-Hsing and the London beast -- were made Friday, Saturday and again yesterday, zoo officials said.

The artificial insemination followed unsuccessful mating attempts Thursday and Friday in which Hsing-Hsing and Ling-Ling continued to suffer from what zoo officials call "ineffective breeding posture."

The pair -- the zoo's most popular attraction with an estimated 3 million yearly visitors -- has been unsuccessful in six years of attempted mating.

A 1980 attempt at artificial insemination was unsuccessful, but zoo officials this year hedged their bets by using the multiple sperm samples, Reed said. He said this was necessary because male pandas are fertile only during a limited season and the quality of the sperm is unpredictable.

The London samples were taken from Chia-Chia, a male panda who was brought here last year in an unsuccessful mating attempt.

A team of anesthesiologists, semen specialists, veterinarians and other specialists made an incision in Ling-Ling's abdomen and used an optical device called a laparoscope to examine her ovaries to determine when she was ovulating. An electronic device was used to extract the sperm from Hsing-Hsing, which they administered to Ling-Ling over a three-day period in an effort to maximize chances of impregnation, zoo officials said.

Results will not be known for about four months. According to zoologists, it is very difficult to tell if pandas are pregnant because adults often weigh more than 250 pounds and show no signs of pregnancy while carrying fetuses that weigh less than a pound.