OT is synthetized in the hypothalamus. These neurons project to the posterior lobe of the pituitary and the external zone of the median eminence. In order to localize OT in the male rat anterior pituitary we have used immunocytochemistry on ultra-thin sections in target cell(s) obtained by cryoultramicrotomy. OT-like immunoreactivity was observed in lactotropes only. No immunoreactivity was observed in gonadotropes, somatotropes, corticotropes or thyrotropes. In lactotropes, immunoreactivity was localized at the plasma membrane level, in the cytoplasmic matrix and around the secretory granules, but not in the other organelles, and in the nucleus. No reaction was observed by using either non-immune serum or anti-OT serum incubated with OT. No modification of OT-like immunoreactivity was observed by using anti-OT serum incubated with heterologous peptides. These results 1) provide immunocytological evidence for the presence of OT in the anterior pituitary gland; 2) indicate the presence of this peptide in one particular cell type, and 3) support the hypothesis that OT could have a direct participation in the regulation of the PRL release.