Adult female rhesus monkeys were given cyproterone acetate orally in doses of 0.04, 0.4, 4 and 40 mg per kg per day for 12 weeks. Its effects were assessed on serum prolactin (PRL) concentration, the morphology of the PRL cells, and the development of the mammary glands. Serum PRL was relatively unchanged in the control animals from the fourth through the twelfth weeks of the study. In contrast, PRL was significantly elevated in each group of drug-treated animals during the same time periods. There was no development of the mammary glands nor was there any evidence of milk secretion in the control animals; however, in the monkeys given cyproterone acetate the mammary glands had extensive lobuloalveolar growth and milk-like secretion that could be expressed as early as the fourth or fifth week of the study. By immunocytochemistry and differential light microscopic staining techniques, the PRL cells in the pituitary glands of the experimental animals were found to be more numerous and much larger than those present in the controls. They displayed a well developed Golgi complex and had an abundance of cytoplasmic RNA. These data suggest that PRL secretion is markedly enhanced by cyproterone acetate.