The present paper describes the fine structure of the accessory sex organs of the male rat as seen after administration of the anti-androgenic compound chlormadinon acetate for 20 days. There was a general involution of the organs with macroscopic atrophy and reduced amount of secretory material. Ultrastructurally the cells contained less organelles as compared with the controls, loss of cytoplasm and reduction both of cell height and width. The major changes also included reduction of the Golgi areas and the rough endoplasmic reticulum. Within the dorsal lobe prominent nuclear changes were found. The alterations observed in the present study are similar to the changes which were found in rats treated with another anti-androgenic compound, Cyp A. It is concluded that these two anti-androgenic compounds exert their effects through similar mechanisms with a specific influence on the prostate and the seminal vesicles.