Endogenous SS14- as well as SS28-like immunoreactive materials were detected in both male and female rats by radioimmunoassay and by immunocytochemistry on ultrathin frozen sections. The content of somatostatin-like immunoreactivity was 0.39 +/- 0.08 pg per mg adenohypophysis. Immunoreactivity was localized by immunocytochemistry in three pituitary cell types: somatotrophs, lactotrophs and thyrotrophs, but not in corticotrophs and gonadotrophs. In these three pituitary cell types the SS28- and the SS14-like immunoreactive materials were localized in the cytoplasm and in the nucleus. In the cytoplasm the immunoreactivity was seen in the cytoplasmic matrix and in the secretory granules. In the nucleus it was present mainly in the euchromatin close to the heterochromatin. In somatotrophs and lactotrophs, SS14- and SS28-like immunoreactive materials have been detected at the plasma membrane level. These results suggest that (1) endogenous SS14 and SS28 are present in adenohypophysis in somatotrophs, lactotrophs and thyrotrophs, and (2) the two peptides act on both the cytoplasmic components and the nucleus.