National zoo officials, concerned over giant panda Ling-Ling's failure to go into heat, are giving her a series of hormone injections to induce her heat cycle so she can mate naturally.

This is the first time Ling-Ling, who is approaching 16 years of age, has missed her heat cycle, which usually occurs in mid-March, and zoo officials started "follicle-stimulating hormone" (FSH) injections on Wednesday, according to Devra Kleiman, the zoo's director of animal programs.

Ling-Ling underwent a physical examination on Wednesday that showed she was in good health but had not ovulated, Kleiman said. Veterinarians began treatment immediately, she said, adding that it will not be harmful to Ling-Ling's health.

"We don't have very many years of mating of Ling-Ling left and we decided to induce a normal heat cycle by using hormone injections derived from the pituitary gland of the brain, to stimulate ovulation," Kleiman said. The injections are given twice daily, once in the morning and once in the evening, and will end on Sunday, she said.

"If it works, we'll expect to see some behavioral changes early next week or late weekend," Kleiman said, the changes being Ling-Ling may start getting aggressive with the zoo's male giant panda, Hsing-Hsing, and her appetite may diminish. She said Ling-Ling and Hsing-Hsing will have between 24 and 36 hours to mate, if the treatment works.

Kleiman said she thinks FSH injections have never before been used on pandas. "We are shooting in the dark but there is a certain amount of information which shows it FSH will produce results," she said.

If the treatment does not work, Kleiman said, zoo authorities will not be overly concerned. "We can have a normal heat cycle next year," she said.

Ling-Ling and Hsing-Hsing's mating progress is watched intently each spring. The pandas were a gift from China in 1972 and for 13 years, the zoo has been trying to breed pandas.

The pandas mated successfully in 1983 and a film of the event was shown on local television. On July 21 of that year, Ling-Ling gave birth to a 4.5-ounce cub but it died three hours later of respiratory failure. Another cub was stillborn on Aug. 6 last year.