Vasopressin, oxytocin and neurophysin were localized in the brains and spinal cords of four primates (tree shrew, squirrel monkey, rhesus monkey and human) using antisera to these peptides and the unlabelled antibody-enzyme peroxidase antiperoxidase method. Magnocellular neurons in the hypothalamus stained positively for vasopressin, oxytocin or neurophysin. Parvocellular neurons of the suprachiasmatic nucleus (SCN) stained positively for vasopressin and neurophysin but not for oxytocin, in all four species. Magnocellular oxytocin, vasopressin and neurophysin neurons project to permeable capillaries in the neurohypophysis, as well as to various extrahypothalamic neural target areas including the central amygdala, nucleus of the solitary tract, dorsal motor nucleus of the vagus, lateral reticular nucleus, dorsal horn, central grey and intermediolateral nucleus of the spinal cord. In target areas, terminals contact somata and dendrites. Parvocellular vasopressin and neurophysin neurons of the SCN do not project to the neurohypophysis, and project only to neural target areas, including the lateral septum, mediodorsal thalamus, lateral habenula, mesencephalic central grey, medial amygdala and ventral hippocampus. (Due to the relatively poor tissue preservation in human autopsy specimens not all projections found in the other primates could be confirmed in humans.) These findings confirm and correlate well with previous descriptions made in rodents, and indicate that vasopressin, oxytocin and neurophysin projections to neural targets are present in primates. Peptides released from these projections probably do not enter the bloodstream, but are rather involved in neural mechanisms.