SUMMARY Male rats injected with 10 mg. cyproterone acetate daily for 28 days were given weekly mating tests with receptive females. At the conclusion of testing the animals were killed and the testes and seminal vesicles were weighed. These organs and the glans penis were then prepared for microscopic analysis. The experimental treatment had no detectable effect upon copulatory behaviour nor upon the testis or the cornified papillae in the integument of the glans penis. The seminal vesicles of treated males appeared to be non-secretory and were lighter than those of control rats but heavier than the vesicles of untreated castrates. It is concluded that although cyproterone acetate partially blocked the action of endogenous androgen on the seminal vesicles it had neither a blocking effect on the penile papillae nor on the central nervous mechanisms which mediate mating responses.