It has been shown that mice transgenic for human GH-releasing hormone (GRH) develop hyperplasia of pituitary somatotrophs, lactotrophs, and mammosomatotrophs, cells capable of producing both GH and PRL, by 8 months of age. We now report that GRH transgenic mice 10-24 months of age develop pituitary adenomas, which we characterized by histology, immunohistochemistry, in situ hybridization, and electron microscopy. Of 13 animals examined, all developed GH-immunoreactive neoplasms that had diffuse positivity for GH mRNA by in situ hybridization. Eleven also contained PRL immunoreactivity; in situ hybridization demonstrated focal PRL mRNA in 3 of 5 immunohistochemically positive tumors. Alpha-Subunit was positive by immunohistochemistry in 8 adenomas, and TSH beta was localized in tumor cells of 5 adenomas. The adenomas had variable ultrastructural appearances, ranging from cells that resembled somatotrophs or mammosomatotrophs to cells with features of the glycoprotein hormone cell line. These findings provide conclusive evidence that protracted GRH stimulation of secretory activity can result in proliferation, hyperplasia, and adenoma of adenohypophysial cells.