The National Zoo said yesterday that it is assembling a panel of experts "to take a hard look" at the deaths over the weekend of two red pandas, which died after rat poison pellets that emit toxic fumes were buried in their yard for the first time.

The adult male pandas, members of an endangered species, were found dead in their yard Saturday morning, less than 24 hours after pellets of aluminum phosphide were buried there to control a burgeoning rat problem. Three zoo employees who went inside the animals' yard experienced headaches, nausea and diarrhea and were treated at a hospital and released.

The zoo said it has asked the Smithsonian Institution's Office of Safety and Environmental Management to select people from inside and outside the museum organization to serve on the panel looking into the deaths. A preliminary necropsy was conducted on the red pandas, but the lab results will not be known for several weeks, said zoo spokesman Robert Hoage. Red pandas are unrelated to the larger, more familiar giant panda.

Aluminum phosphide, a highly toxic substance that reacts with groundwater to produce phosphine gas, has been used in open areas at the zoo for about a year, with no ill effects on zoo animals or humans, Hoage said. This was the first time it was used in an animal exhibit.

Because of its acute inhalation toxicity, the substance must be handled by a licensed pesticide operator. The zoo hired a contractor to perform the job, which was monitored by a zoo employee, Hoage said.

The zoo has trapped rats, shot them with pellet guns and used traditional rat bait in several areas at the zoo, but those methods were only partially successful, officials said. They decided it was safe to bury the poison pellets in the red panda yard because the animals live in trees and seldom venture to the ground. Also, the phosphine gas emitted is heavier than air and is supposed to sink and settle into rat burrows.

Richard Farinato, director of captive wildlife programs for the Humane Society of the United States, said zoos commonly have rodent control problems -- but should never put poison in an animal's exhibit.

"Common sense says you don't use a toxic substance in the same area the animal is in, regardless of the method," Farinato said. The usual practice for rat control at zoos, he said, is to put out bait pellets that the rats eat, but the pellets are never put in an animal's pen.

The red panda deaths followed the recent deaths of five other animals at the zoo: a lion, a white tiger, two adult giraffes and a seal.

The tiger was euthanized because of age-related osteoarthritis, and the seal died of heart disease. The giraffes died within a seven-month period last year because of age-related digestive problems, prompting zoo officials to consider whether changes in their diet or dental care might have prolonged their lives.

Zoo officials said last week that a lion that died in October suffered complications from anesthesia administered so that veterinarians could conduct an exam to see why the animal was limping. The lion, a 14-year-old male named Tana, appeared to be recovering but was found dead the next morning, according to a report by zoo pathologist Richard J. Montali. In zoos, lions can live as long as 25 years.

In the winter of 2000, two young zebras died at National Zoo facilities after keepers failed to feed them enough fat and protein and keep them sufficiently warm on frigid nights. Postmortem exams indicated that both animals died of the cold weather and had all but depleted their stores of fat.

Farinato also criticized the zoo because of "the string of deaths," particularly those of the lion and the giraffes.

"This is another instance where through their own blundering, they've killed an animal," Farinato said. "For an institution that's been around as long as the National Zoo has, and with a staff that's supposed to be so highly trained, things like this shouldn't be happening."

Staff researcher Carmen Chapin contributed to this report.