The tissue-specific expression of a fusion gene encoding the mouse metallothionein-1 promotor and the coding region of the human GH-releasing hormone (hGRH) gene was studied in transgenic mice by immunohistochemistry using an anti-hGRH serum that does not recognize endogenous mouse GRH. hGRH immunoreactivity (GRH-IR) was detected in specific cells of the pituitary, pancreas, kidney, duodenum, lung, testis, ovary, adrenal, heart, and brain. In the pituitary, using double immunofluorescent staining, GRH-IR was found in some, but not all, somatotrophs, gonadotrophs, thyrotrophs, and mammotrophs. GRH-IR was found in both pancreatic exocrine cells and endocrine islets. Within the islet, GRH-IR was colocalized in A and D cells with glucagon and somatostatin, respectively. Immunopositive cells in other tissues were localized in kidney proximal convoluted tubules, duodenal submucosal glands of Brunner, the smooth muscles of pulmonary arterioles, testicular Leydig cells, oocytes, adrenal medullary chromaffin cells, and cardiac atria. In the brain, GRH-IR was seen in the external layer of the median eminence and in perikarya and fibers of the hypothalamic arcuate nucleus, the parvocellular region of the paraventricular nucleus, the supraoptic nucleus, and the amygdala. Somatostatin-immunoreactive cell bodies and fibers in transgenic and control mouse hypothalamus were not appreciably different. In summary, hGRH expression in transgenic mice occurs in a cell-specific manner in the hypothalamus as well as in numerous other tissues, many of which have secretory functions.