To evaluate the origin of seminal prostaglandins, genital tract fluids were analysed for prostaglandin content in two clinical situations. Six patients delivered semen samples before and after vasectomy. In these patients the prostaglandin concentration remained essentially unchanged although sperm density decreased to zero. In another patient secretory products from the testis and epididymis, and the ejaculate representing mainly the secretion of the seminal vesicles and the prostate gland, were collected separately. Secretions obtained from the testis and epididymis did not contain detectable amounts of prostaglandins, whilst in the ejaculate from the same patient the concentration was within normal limits. The results of the study show that the testis and epididymis do not contribute significantly to the prostaglandin content of human seminal fluid. The previous assumption that the seminal vesicles are the main source of seminal prostaglandins is thus supported.