The presence of parvocellular vasopressin- and neurophysin-containing neurons in the suprachiasmatic nucleus (SCN) was investigated in 13 mammalian species representing six mammalian orders (marsupials, rodents, lagomorphs, artiodactyls, carnivores, and primates), using specific antisera to vasopressin and neurophysin in the unlabelled antibody=enzyme immunoperoxidase method. In all mammals examined, including man, parvocellular vasopressin and neurophysin neurons were found in the SCN. Only a portion of SCN neurons contain vasopressin and neurophysin, the number varying with species. Cell counts comparing the number of immunoreactive to Nissl-stained neurons showed averages of 17% immunopositive neurons in the rat SCN, and 31% in the human SCN. No oxytocin-containing SCN neurons were observed. These findings suggest that parvocellular vasopressin and neurophysin neurons are widely represented in mammals.