Pituitary GH cells from adult male Rana perezi frogs were investigated in vivo and in vitro after stimulation with synthetic thyrotropin-releasing hormone (TRH). The volume density (Vv) of the secretory granules (SG), rough endoplasmic reticulum (ER), and Golgi complex (GC), and the numerical density (Nv) of the granules were estimated by ultrastructural morphometry. GH-producing cells were identified by the immunocytochemical colloidal-gold method, using anti-ovine-GH as primary antiserum. The animals involved in the in vivo experiment were given daily injections of synthetic TRH into the dorsal lymph sac. In vitro, hemipituitaries were cultured in a superfusion system. TRH caused cytological changes in GH cells both in vivo and in vitro. In vivo, GH cells showed a 27% decrease in the Nv of the SG after 8 hr of treatment and an increase Vv of the GC (1.6 fold) and ER (2.7 fold) after 48 hr of treatment compared to the cells in control animals. Cells tended to recover control values after 6 days. The in vitro administration of TRH induced a 48% decrease in the number of SG in GH cells after 24 hr, although the development of the biosynthetic machinery (GC and ER) was not enhanced. These results clearly indicate that, at the dose used in vitro, TRH only stimulates the release of GH in the short-term while, in vivo, it promotes long-term synthesis of new hormone. The data obtained suggest that TRH modulates the secretion of GH in amphibians by acting directly upon GH cells, while the effect on the synthesis might be mediated by other hypothalamic factors influenced by TRH.