Recent studies have suggested zinc to be a possible modulator of hormone secretion from the anterior pituitary. The three aims of the present study were: to estimate the total amount of zinc in the gland by instrumental analysis; to visualize the zinc at light and electron microscopical levels, and to estimate the amount of zinc that can be visualized by histochemical techniques. PIXE measurement showed the total amount of zinc to be 74 ng/mg dry weight in males and 100 ng/mg dry weight in females, which is a highly significant difference. Histochemically, the zinc was shown by a modified Timm method and the selenium method to be localized to the secretory granules, and to a smaller extent to the Golgi apparatus of the somatotrophs, corticotrophs and thyrotrophs. Chelation of tissue zinc by intravital dithizone treatment effectively blocked subsequent selenium and Timm staining in the secretory granules, and PIXE assays of the chelated metal (extracted into CCl4) showed it constituted less than 5% of the total zinc in the tissue. It is concluded from the study that zinc is present in the anterior pituitary of rats in rather high amounts and that loosely bound zinc, which is suggested to exert a biological effect by itself, can be located to the parts of the cells responsible for production, storing and release of hormones.