The adenohypophysis occasionally contains cysts of epithelial nature. The present study describes the incidence and immunohistochemical characteristics of these epithelial cysts in the adenohypophysis of rats at different ages and in young mice. Epithelial cysts were found in about 10% of the partes distales irrespective of age and animal species. Their incidence in the pars tuberalis was higher: 22% and 58% in young rats and mice, respectively. Immunohistochemically, cells composing these cysts failed to contain S-100 protein. Although cysts found in the pars tuberalis frequently possessed immunoreactive luteinizing hormone-producing cells, most cysts in the pars distalis were immunonegative when stained with antisera to several different adenohypophysial hormones. Examination of fetal rat hypophysis has shown a close topographical relationship between cysts and the pharyngeal duct. This fact, together with the frequent occurrence of cysts in the ventro-medial region of the pars distalis in neonatal and young animals, indicates that these cysts are probably derived from a part of oral epithelium that is otherwise destined to degenerate at the time when Rathke's pouch closes.