Prostaglandin F (PGF) was measured in the testes, epididymides, vasa deferentia, seminal vesicles, coagulating glands, ventral prostate and dorsolateral prostate of adult male rats. Most of the PGF present in the vas deferens was located in its luminal contents but apparently was not associated with spermatozoa. The secretions of seminal vesicles and coagulating glands contained very little PGF. Castration reduced PGF concentration in the vas deferens and seemed to have caused a similar change in the epididymis and the seminal vesicles. Administration of testosterone propionate to castrated rats increased concentrations of PGF in the vas deferens, seminal vesicles and apparently in the epididymis. These observations suggest that the synthesis of PGF in the reproductive system of the male rat is under androgen control. In mice, concentrations of PGF in the epididymis and in the seminal vesicle were increased after castration. However, in both rats and mice the content of PGF in these tissues was reduced in long-term castrates. (Endocrinology95: 1739, 1974)