Prostaglandin synthetase activity is histochemically detected in the epithelial cells of the human prostate, seminal vesicle, and deferential ampulla. The addition of the specific inhibitor indomethacin to the incubating medium strongly reduces the staining reaction. The presence of exogenous substrate in the medium is not required for the reaction, since identical results are observed following incubation with and without arachidonic acid. The presence of the enzyme appears to be related to the secretion of prostaglandins into the seminal fluid, but there is a possibility that some prostaglandins influence the metabolism of the secreting cells themselves. The localization of prostaglandin synthetase in the ductal epithelia of human salivary glands is related to a possible regulatory role of the prostaglandins upon the reabsorptive activity of the ductal cells.