Brooks, a 21-year-old lowland gorilla at the Cleveland Metroparks Zoo, died after he was anesthetized for an examination.
Veterinarians had anesthetized the gorilla so they could check him for a persistently swollen tongue that was making it difficult for him to swallow. The 380-pound gorilla had trouble breathing soon after he was injected, and then his heart stopped beating on Thursday, zoo officials said.
Results of an examination to determine the cause of death should be available in about two weeks, said zoo spokeswoman Sue Allen.
Brooks was one of three male gorillas that came to the zoo in 1994 from Chicago's Lincoln Park Zoo. The trio was among the country's first "bachelor groups," bands of all-male gorillas designed to better manage the growing captive gorilla population.
Gorillas operate in harems with one sexually mature male and a group of females. As more gorillas were born in captivity, zoos created all-male groupings to handle additional adult males.
Brooks was the dominant male in the trio that included Bebac and Mokolo.
"He used to like to sit around and look regal," said Tara Turner, who manages donor programs for the Cleveland Zoological Society.
Brooks had been in relatively good health until recently, when his tongue began to swell and he had trouble breathing. Zoo veterinarians gave him antibiotics and anti-inflammatory medications, but the swelling began again when they took him off the medication.