Two red pandas were found dead yesterday morning inside their exhibit at the National Zoo and three employees who went inside the animals' enclosure fell ill, according to zoo officials who said they are investigating whether rat poison used at the zoological park may have played a role.

The adult male pandas were found on the ground in their outdoor exhibit at 8:50 a.m. by a zookeeper conducting routine morning checks, officials said. The National Zoo's red pandas -- about the size of large house cats -- are unrelated to the better-known and much larger giant pandas. They were 7 1/2 and 5 1/2 years old and had no record of medical problems. Red pandas often live into their early teens while in the care of zoos.

Zoo spokesman Robert Hoage said a zookeeper and two mammal curators went into the pandas' enclosure at midafternoon yesterday to help remove the pandas and soon began to experience headaches, nausea and diarrhea. The employees were taken to a hospital, where they were examined and released. D.C. crews dealing with hazardous materials later tested the exhibit area for toxic fumes but found nothing, Hoage said.

"We know [the zoo employees] were upset by the events, but we don't know if their symptoms were related to the [animals'] deaths," Hoage said.

Officials said they were investigating whether a new rat control method being used at the zoo -- using poisonous pellets buried deep in the ground -- could have contributed to the animals' deaths. Hoage said the rat bait is used under "tightly controlled" conditions, but the possibility of poisoning exists and authorities were looking into it.

The developments followed several recent deaths among the zoo's older animals, including a lion, a white tiger, two adult giraffes and a seal. On Friday, zoo officials said the lion died from complications from anesthesia administered during a medical exam. The tiger was euthanized because of age-related osteoarthritis. The giraffes suffered from digestive problems, and the seal died of heart disease.

A preliminary necropsy was conducted on the red pandas yesterday. A final cause of death will not be known for several weeks, Hoage said.

The zoo was holding the red pandas in nonbreeding status for its species survival program. The zoo has two other red pandas at its conservation center in Front Royal, Va.

The deaths came a day after officials revealed that the lion died from complications of anesthesia administered in October so that veterinarians could diagnose the cause of his lameness.

The 14-year-old lion, Tana, was glassy-eyed and dopey from the anesthetic when keepers left the zoo at 6:30 p.m. Oct. 10, but he appeared to be recovering normally, according to a report by zoo pathologist Richard J. Montali. The 461-pound animal had been given a drug to reverse the anesthesia's effects. A keeper found him dead the next morning. In zoos, lions can live as long as 25 years.

Montali's report said the cause of death was fluid buildup in the lungs associated with the anesthesia. A pool of frothy, bloody fluid was found near the lion's carcass. The lion, who was late middle-aged for a captive animal, was in good condition otherwise. Neither the physical exam nor the necropsy discovered the cause of his lameness.

Montali said Friday that some anesthesia remained in the lion's system and was likely reactivated, making him drowsy. "Then his airway got into trouble -- how, we don't know," he said. His report said the lion's kidneys had significant levels of the sedative xylazine 18 hours after it was administered.

The zoo's chief veterinarian, Suzan Murray, administered the anesthesia, with her predecessor, zoo Director Lucy H. Spelman, and another veterinarian standing by. Murray said she is puzzled and frustrated because the zoo's investigation has not discovered what aspect of the anesthesia procedure led to Tana's death.

She said that the anesthetic is by far the safest for big cats and that she has no plans to change it. Nor will she recommend any change in protocol -- such as checking animals throughout the night after anesthesia -- until she is confident that it would have prevented the death.

"If I had a sense of what went wrong, I'd have a better idea of what to recommend," Murray said. "Hopefully, we will find out how and when, but we don't always."

Murray and other zoo officials said they anesthetize animals only when they must because it is inherently risky. Murray said she was checking with other animal care facilities to see whether they have had similar problems. But she pointed out that Tana had previously been anesthetized six times without problems. The procedure has been carried out successfully on other zoo lions and tigers.

Zoo officials also released reports Friday about the September death of an elderly gray seal and the October death of an 18-year-old white tiger. The 23-year-old seal, Keltie, died of heart disease, but West Nile virus was a contributing factor. The tiger, Taj, was euthanized because he could barely move and was in great pain. The animal's necropsy confirmed severe osteoarthritis.

Red pandas are about the size of a large house cat and are unrelated to the giant panda.Tana, a 14-year-old lion, died in October, one of several recent deaths at the zoo, including those of an elderly gray seal and an 18-year-old tiger.