The tetradecapeptide, somatostatin (SRIF), is a potent inhibitor on pituitary hormone release, by a direct effect. The immunocytological method was used with the aim of localizing SRIF at the cellular and subcellular levels. Rat pituitaries were fixed and frozen. Ultrathin sections, obtained by cryoultramicrotomy, were incubated with anti-SRIF serum. The antigen-antibody reaction was detected by 4-chloro-1-naphthol. SRIF immunoreactivity was observed in somatotrophs, thyreotrophs and prolactin cells, but not in corticotrophs or gonadotrophs. In immunoreactive cells, SRIF was found in the cytoplasmic matrix, in the secretory granules and in the nucleus distributed primarily in the euchromatin, in the vicinity of the heterochromatin regions. SRIF immunoreactivity was also observed at the plasma membrane. No immunoreactivity was observed when nonimmune serum or anti-SRIF serum incubated with SRIF was used. No modification was observed when anti-SRIF serum incubated with gonadoliberin, thyroliberin or vasopressin was used. These data (1) provide immunocytological evidence for the presence of somatostatin in pituitary gland, and (2) indicate the presence of SRIF peptide in the somatotrophs, thyreotrophs and prolactin cells.