Despite its occurrence in most vertebrate species, the function of the hypophysial pars tuberalis (PT) remains obscure. Recent immunocytochemical studies have demonstrated the presence of hormone-containing cells in the few species studied. In the present study the secretory cell composition of the PT was characterized in a variety of mammals using immunocytochemistry. Species studies were the mouse, rat, guinea pig, rabbit, sheep, rhesus monkey, baboon, and human. Antisera were chosen on the basis of their ability to identify a distinct cell population in the pars distalis. A total of 21 antisera were used to identify GH, PRL, ACTH, beta-endorphin, LH, FSH, and TSH. Gonadotropes were identified in the PT of all eight species and were the predominant immunoreactive cell type in the human, baboon, rhesus monkey, sheep, guinea pig, rabbit, and mouse. Thyrotropes were detected in all species except the sheep. They were the predominant cell type in the rat but were less common than gonadotropes in other species. No other secretory cell types were found, with the exception of occasional somatotropes and mammotropes in some human specimens, and small clusters of opiocorticotropes in the guinea pig. Thus the general pattern in the mammalian PT is the presence of gonadotropes and thyrotropes and the absence of other pituitary cell types. In the human, baboon, and rat, all PT parenchymal cells can be identified immunocytochemically. However, in the rhesus monkey, sheep, guinea pig, rabbit, and mouse, the majority of PT cells do not react with any antisera, and thus their function is unknown. Follicles are common in the PT of most mammalian species, however, the luminal contents do not react with antisera to adenohypophysial hormones.