Estrogen affects the synthesis and release of several pituitary hormones. The estrogen receptor (ER), a member of the steroid hormone receptor family, is thought to mediate transcriptional effects in a cell-specific fashion. We investigated whether ER is expressed in specific hormone-producing cell types in the human pituitary and its adenomas. Pituitary adenomas (n = 34) were collected at the time of surgery, and normal glands were obtained from autopsy. Expression of ER messenger ribonucleic acid (mRNA) was determined by reverse transcription-polymerase chain reaction (RT-PCR) and in situ hybridization. ER was also localized with immunohistochemistry and protein extraction. By RT-PCR, ER mRNA was found in the nontumorous pituitary and in pituitary adenomas expressing only PRL, in those producing GH and PRL, and in adenomas expressing the gonadotropic hormones. No ER mRNA was detected in adenomas expressing only GH without PRL or gonadotropins, nor in tumors producing ACTH without PRL or gonadotropins. In situ hybridization was not as sensitive or specific as RT-PCR. Biochemical analysis performed on seven tumors that were positive for ER mRNA by RT-PCR detected ER protein in only one PRL adenoma and one oncocytoma and yielded negative or equivocal results in one PRL adenoma, three GH-PRL adenomas, and one null cell adenoma. ER protein was localized by immunohistochemistry in scattered cells of the nontumorous adenohypophysis and in a few PRL and gonadotroph adenomas. We conclude that ER expression, as determined by RT-PCR, correlates with the expression of PRL or gonadotropins; in contrast, ER mRNA was not detected in adenomas that express only GH or ACTH. These findings implicate ER as a cell-specific transcription factor that may regulate cytodifferentiation in the pituitary.