The mouse epididymis was studied to localize histochemically a number of hydroxysteroid dehydrogenases in the various zones. The epithelium of the posterior half of the initial segment (head) and the anterior half of the middle segment (body) shows a strong reaction for △5-3β-, 3α,5α-, 3α,5β-, 11β-, 16α-, 170β- 20α- and 20β-hydroxysteroid dehydrogenases. This activity attenuates posteriorly. Only the 11β-hydroxysteroid dehydrogenase is present throughout the length of the epididymis. The luminal contents of the middle segment also show the histochemical utilization of a number of steroids.