Luteinizing hormone releasing hormone (LHRH)-treated adenohypophyses from castrated, cyproterone acetate-injected adult male mice were studied with transmission electron microscopy (TEM). LHRH-treated luteinizing hormone (LH) gonadotrophs (types a and b) in castrated mice showed accumulation of secretory granules at the capillary pole, dilated Golgi apparatus and extensive rough endoplasmic reticulum (RER) profiles. Following treatment, granular content in LH type-a gonadotrophs was decreased and profiles of secretory granules were seen in the intercellular and perivascular spaces. Some RER cisternal hypertrophy was noted, and the Golgi apparatus was dilated. In LH type-b gonadotrophs there were secretory granules lined along the plasma membranes, several lysosomes, multivesicular bodies, and short RER profiles. LHRH stimulated secretory granules release of LH gonadotrophs in cyproterone-acetate-treated castrated mice. Cyproterone acetate may have blocked hypothalamic LHRH stimulation of pituitary LH gonadotrophs.