DALLAS - Hercules, a 43-year-old gorilla at the Dallas Zoo, died from cardiac arrest Wednesday afternoon after undergoing treatment to relieve back pain caused by arthritis.

"He's been suffering from severe arthritis for a number of years," said Susan Eckert, a Dallas Zoo spokeswoman. "He was in so much pain that he couldn't lift his arms to feed himself, and he couldn't move or walk."

Veterinary doctors performed CPR on the gorilla, but he died about 4 p.m. at the zoo's hospital.

Ms. Eckert said the multi-step procedure that doctors performed included a combination of massage and steroid treatment to relieve the arthritis in his back.

Ms. Eckert said doctors felt the procedure was necessary because Hercules was immobile due to his arthritis.

"That's [cardiac arrest] a risk that you have when you do any type of veterinary procedure and you have an animal sedated," she said.

A necropsy, an autopsy performed on animals, was performed on Hercules today to determine the exact cause of death.

Hercules was regarded as a friendly and popular animal, often sitting at his exhibit's front window, where patrons could see him.

"He'd look at you from the side of his eyes, which is good gorilla manners," Ms. Eckert said. "In the gorilla world, you don't make full eye contact.

"He seemed to enjoy looking at the children through the window. He'd sit there and look at them and sometimes put his hands out. He was a sweet gorilla."