Testes were obtained at post mortem from three Western lowland gorillas, two Bornean orang-utans and one Chimpanzee. All specimens were adults which had died in captivity. Histological examination revealed that two gorillas exhibited complete degeneration of the seminiferous tubules and that the testes contained large amounts of interstitial tissue. Some, less obvious, signs of degeneration were noted in the third specimen but poor preservation of the material hampered interpretation of these features. The testes of the Orang-utans and Chimpanzee were structurally normal. Testicular atrophy in captive gorillas may be more common than is generally realized and there is a need for research on fertility and reproductive endocrinology of male specimens. At the moment it is not possible to identify the causes of testicular dysfunction in captive gorillas.