A high incidence of adrenal tumors was observed in aged female NIH Swiss mice which had been ovariectomized at 2 to 4 weeks of age but not in nonovariectomized controls. Although tumors weighing more than 1 g were not infrequent in the oldest (> 24 months) animals, adrenal glands did not appear macroscopically abnormal before the age of 18 months. Histologically, however, focal or diffuse abnormalities were found in essentially every gland examined from mice over 12 months of age, including glands of normal size. Since the NIH Swiss mouse has been shown to contain an endogenous xenotropic virus whose expression is under hormonal control, the adrenal tumors were examined in detail for evidence of abnormal viral expression. We were unable, by a variety of techniques, to demonstrate elevated expression of type C virus in these adrenal tumors.