The localization of 5 alpha-reductase was immunohistochemically studied in the anterior pituitary of male rats, using a polyclonal antibody against 5 alpha-reductase rat type 1. The immunoreactive cells were concentrated in the central region and on the border of the intermediate lobe in the anterior pituitary, but not in the intermediate or posterior lobe. The immunoreaction was located mostly in the cytoplasm and occasionally in the cell nuclei. The immunoreactive cells showed alterations in size and number and in the intensity of the immunoreaction after gonadectomy. One week after castration, the cells became larger and the immunoreactivity increased. Two weeks after castration, the number of immunoreactive cells increased. Double immunostaining using anti-luteinizing hormone beta-subunit or anti-follicle stimulating hormone beta-subunit antibody revealed that most of the cells containing 5 alpha-reductase were gonadotrophs. Electron microscopically, the immunoreactive cells showed lamelliform rough endoplasmic reticulum and a depletion of secretory granules 1 week after castration. One week later, the rough endoplasmic reticulum was developed and dilated and the number of secretory granules increased. These results suggest that 5 alpha-reductase is located in the gonadotrophs of rat anterior pituitary and that it is involved in the feedback regulation of gonadotropin secretion by androgens.