Toni the elephant lived alone for years in a decrepit concrete enclosure at a zoo in Pennsylvania. She was afraid to step on dirt and could not communicate with other elephants, much less socialize with them. When she was moved to the National Zoo in 1989, she was accompanied by a car caravan of fans who later rejoiced as she adapted well to her new, improved home.

Now Toni, one of four Asian elephants at the animal park and a favorite of keepers and the public, is facing a new challenge. Nearly 40, she is undergoing treatment for arthritis in her legs, a serious condition that could eventually require the zoo to euthanize her.

"We're happy with how she is doing" with treatment, said Tony Barthel, assistant curator for elephants, watching her in the yard yesterday. "But we don't know her rate of decline in the future or her level of discomfort."

To ease the inflammation in her joints, the ailing elephant is receiving 9,000 milligrams of ibuprofen twice a day. A typical human dosage is closer to 200 to 800 mg. The zoo's veterinarians are monitoring her condition and collecting blood and urine samples weekly. And keepers are bathing her on a cushioned floor and improvising other ways of tending to Toni so that she doesn't have to lie down, a particularly painful movement for her these days.

"We're seeing physical and behavioral changes that have us really concerned," said Marie Galloway, the zoo's elephant manager.

Zoo staff said they noticed problems earlier in the summer. Toni, who injured her left front leg in 1975 while housed at a zoo in Scranton, Pa., used to place more weight and pressure on her right leg. But she moves more slowly and has started shifting her weight to her injured left leg, indicating a more widespread arthritic condition, Galloway said.

Sometimes, Galloway and Barthel said, the elephant will even back up and lean against a wall to take the pressure off both front legs.

Despite her decline, zoo staff said Toni is enjoying life. She has a good appetite and enjoys taking dips in the pool and interacting with the zoo's three other female elephants. But the zoo's young male elephant, Kandula, who is almost 4 years old and weighs more than 3,000 pounds, is being kept away.

"He's too rambunctious for Toni," Galloway said.

Elephants, accustomed to roaming 40 miles a day in the wild, often develop foot problems and arthritis in captivity, where the floors of their enclosures frequently have hard surfaces. The zoo's Elephant House has some rubberized flooring, and the elephants spend a lot of time in their yard. A planned new elephant facility will have a more comfortable surface and more space, zoo officials said.

Toni has bounced back in the past. The elephant was successfully treated in 2001 for kidney disease, said Suzan Murray, the zoo's head veterinarian. The zoo published a scientific paper that outlined the course of treatment.

"She has a fighting spirit," said Murray, noting that the zoo hopes the same team management approach to Toni's care will prolong her life in reasonable comfort.

Toni was born in Thailand and brought to the United States when she was about 7 months old. When the Scranton zoo closed, Hank Robinson, a private citizen who loved the elephant, paid for her shipment to the National Zoo. He makes regular trips to visit her with others from Scranton, Galloway said.

"We saw the opportunity to dramatically improve her life. We knew we could make a difference," said Galloway, who wants the zoo to develop a "multi-generational" herd of elephants when it gets its new facility.

Galloway said Toni was in very bad shape when she arrived, with the leg injury and a half-inch of dead skin. She was afraid of dirt surfaces, didn't know how to swim and didn't know how to vocalize with other elephants.

With training, Toni adapted well to her new environment, became friends with the other elephants and is now so reliable and easy to work with that the zoo uses her as a "starter elephant" for new trainers.

"She's just a wonderful animal and a joy to work with," Barthel said. "A lot of people are attached to our elephants. . . . [Caring for Toni] could be a long, long story, and that would be great."

"We're happy with how she is doing," Tony Barthel, an assistant curator, says of Toni. "But we don't know her rate of decline in the future or her level of discomfort."