SUMMARY The present paper describes the fine structure of the ventral and lateral prostate and the seminal vesicles of the male rat after administration of an anti-androgen, cyproterone acetate, given as subcutaneous injections for varying periods (5–18 days). The treatment caused a general involution with macroscopic atrophy of the organs and reduced the amount of secretory material. There was a marked reduction of both the cell height and width and loss of cytoplasm. The main cytoplasmic changes consisted of a quantitative reduction of the rough endoplasmic reticulum and free ribosomes; the Golgi apparatus was diminished and the number of secretion granules markedly decreased. All the alterations observed are apparently similar to the changes induced by castration.