The contribution of the suprachiasmatic nucleus to the vasopressinergic innervation of the rat brain was determined by following the changes in the immunocytochemical localization of vasopressin-containing fibers at various intervals between 2 days and 12 weeks after bilateral lesioning of this nucleus. The disappearance of the vasopressinergic fibers makes it plausible that vasopressin-containing pathways run from the suprachiasmatic nucleus towards the periventricular nucleus, the dorsomedial nucleus of the hypothalamus and the organum vasculosum laminae terminalis. Since after lesioning the vasopressinergic fibers remained unaltered in the lateral septum, the lateral habenular nucleus, the nucleus of the amygdala, the diagonal band of Broca, the nucleus of the solitary tract, the interpeduncular nucleus and the dorsal raphe nucleus, the suprachiasmatic nucleus sends no or only minor projections to these areas. In contrast to the literature, these findings indicate that the paraventricular nucleus and possibly the supraoptic nucleus form the major source for vasopressinergic pathways in the brain.