Prostaglandins (PGs) are undoubtedly involved in several aspects of male reproductive physiology, but the uniquely high concentrations in human semen are of particular interest since we are now closer to an explanation of their function. Despite the early work of von Euler, who proposed the prostate as the source of PG in human semen, the evidence is now clear that the seminal vesicles are the main source of PG. Apart from the involvement of PGs in sperm transport from the upper part of the reproductive tract (which would involve PG synthesis in the vasa and the epididymides), there is no evidence to suggest that PGs are involved in the ejaculatory process. The possibility that low testosterone levels could reduce both PGs and fertility cannot be dismissed, as testosterone has a very marked effect on PG secretion in hypogonadal men, and a beneficial effect on sperm motility (presumably indirect, via stimulation of accessory glands) has been reported.