Mopie, a 34-year-old male, collapsed and died Monday at the Great Ape House. The animal was found in 1998 to have a chronic disease of the heart muscle that decreased its ability to pump blood.
Officials were awaiting a final pathology report that might provide more information about the cause of Mopie's death.
A 23-year-old gorilla named Kuja that died Saturday was the only other adult male in National Zoo's collection of seven western lowland gorillas.
Though Kuja's heart function was found to be normal for his age during a routine physical in late March, members of the zoo staff reported the animal was lethargic and experienced appetite loss this month.
A June 20 ultrasound showed that Kuja's heart muscle was unable to contract normally, and his heart's pumping capacity had diminished.
The animals are considered endangered with dwindling wild populations. Zoo officials have said one of the leading causes of death among adult male gorillas is heart disease.