Microscopic analysis of a 38-year-old chimpanzee (Pan troglodytes) revealed a well differentiated Sertoli-Leydig cell tumor (arrhenoblastoma), 4 mm in diameter. Extensive follicular atresia and thecal hypertrophy were noted. The endometrium showed extensive adenomatous hyperplasia, an indication of hyperestrinism. It was suggested that the apparent excessive estrogenic secretion might have originated in the hypertrophic thecal tissue rather than in the tumor.