Adenohypophysial cells having a glucocorticoid receptor were immunohistochemically determined in rats. For detecting the presence of the glucocorticoid receptor, we used the monoclonal antibody for a glucocorticoid receptor (BuGR-2), and immunohistochemically examined the phenotypes of the cells that exhibited BuGR-2-immunoreactivity. The immunoreaction for the glucocorticoid receptor was confined to the nuclei of the majority of corticotrophs (70%) and of some somatotrophs in intact animals. Following adrenalectomy, all corticotrophs became significantly hypertrophic, losing their immunoreactivity for the glucocorticoid receptor. In contrast, somatotrophs that had also lost the immunoreactivity for the glucocorticoid receptor in the nuclei greatly diminished in size. Intraperitoneal administration of corticosterone was performed in adrenalectomized animals to supplement glucocorticoids. This treatment restored BuGR-2-immunoreactivity in the nuclei of some corticotrophs. In intact rats, immunolabeled corticotrophs were classified into two types, stellate and polyhedral. However, the immunoreaction for the glucocorticoid receptor was equally evident in the cell nuclei of these different types of cells. It is concluded that, in rats, both corticotrophs and somatotrophs are target cells of glucocorticoids, although these cell types display opposite growth responses to the removal of glucocorticoids.