The oxytocin and vasopressin nerve fibers in the intra- and extrahypothalamic neuronal systems of several mammalian brains are immunohistochemically demonstrated using a modified peroxidase-antiperoxidase technique. The axonal processes of these peptidergic neurons are classified into thick and thin beaded fibers. Thick beaded fibers were preferentially distributed in the hypothalamo-neurohypophysial tract and in some circum-ventricular organs, with termination on the blood vessels. Thin beaded fibers were found in various extrahypothalamic areas and these terminals were in the vicinity of the neuronal somata of such areas. This report suggests that there are at least two different functions concerning neurotransmission in the oxytocin and vasopressin neuronal system.