Immunocytochemical studies have revealed the presence of extensive vasopressinergic projections from the suprachiasmatic nucleus to the limbic system and other brain areas. Vibratome sections and the unlabeled antibody enzyme method were used to investigate the ontogeny of the vasopressinergic neurons of the suprachiasmatic nucleus and their exohypothalamic fibers in the rat brain. The first immunopositive neurons of this nucleus were revealed on the 2nd postnatal day. An adult appearance of the suprachiasmatic nucleus was detected on day 14. Although fibers appeared on the periventricular nucleus already on the 7th postnatal day, such fibers were visible in the lateral septum and lateral habenular nucleus only on day 10. From the 12th postnatal day onwards a marked sex difference developed with respect to the density of the vasopressin fibers in the lateral septum and, to a lesser extent, in the lateral habenular nucleus. In male rats the fiber density was higher in both areas. This sex difference persisted in adulthood.