Prostaglandins, lipoid substances discovered in human semen as modulators of uterine muscle contractility, are known to play significant roles in virtually all mammalian organ systems, but their male reproductive functions are unclear. Cyclooxygenase, the rate-limiting enzyme in prostaglandin synthesis, occurs in two isoforms distinguished on the basis of constitutive (COX-1) or inducible (COX-2) expression patterns in mammalian tissues. However, in the adult rat male reproductive system, immunohistochemistry and Western and Northern analysis showed that COX-2 is the predominant isoform and is heavily localized to the epithelium of the distal vas deferens, where constitutive expression is manyfold greater than in any other organs of the body. COX-2 is not detected in the proximal one-half of the vas nor in the testis, epididymis, seminal vesicles, or prostate. Elimination of luminal sperm by vasectomy does not affect COX-2 levels, whereas castration severely depletes COX-2 and androgen replacement after castration restores COX-2, indicating that COX-2 expression in the vas is androgen dependent. Because the distal vas also comprises an extensive submucosal venous plexus connected to the penile corpora cavernosa, prostaglandins from the vas may play a role in erection.