The endocrine function of the hypophysial pars tuberalis is currently unknown. Recent immunocytochemical and electron microscopic studies have shown the presence of gonadotropin-producing cells in this tissue in intact rats. Because the pars tuberalis is not destroyed by hypophysectomy, the objective of the present study was to determine whether this gland is activated to produce pituitary hormones following hypophysectomy. Hormone-producing cells were identified by the unlabeled antibody peroxidase-antiperoxidase method of immunocytochemistry using primary antisera generated against the following hormones: LH, FSH, TSH, GH, PRL, ACTH, and beta-endorphin. In intact control rats, the only cell types detected in the pars tuberalis were gonadotropes and thyrotropes. Most gonadotropes contained both LH and FSH. The TSH cells were a separate cell population, and constituted the majority of pars tuberalis parenchymal cells. As early as 1 week after hypophysectomy, a hyperplastic and hypertrophic response was noted in the gonadotropes, and both cell types showed cytological features characteristic of enhanced hormonal synthesis and secretion. These responses increased with time post hypophysectomy. Secretory cell types not present in the pars tuberalis before hypophysectomy, i.e. somatotropes, mammotropes, opiocorticotrophes, were not activated or induced to differentiate after hypophysectomy. Based on these immunocytochemical observations, the hypophysectomized rat cannot be viewed as an animal totally devoid of anterior pituitary hormones. It has the capacity, or at least the potential, for secretion of LH, FSH, and TSH.