Using an affinity-purified antibody directed against the vasopressin-related glycopeptide as a marker to distinguish reliably vasopressin-producing from oxytocin-producing cells, we mapped the distribution of vasopressin cell bodies and processes in the brain of adult, untreated guinea pigs. Strongly labelled cell bodies were detected in the area of the paraventricular, supraoptic and suprachiasmatic hypothalamic nuclei. Lightly stained cell bodies were observed in the zona incerta, in the bed nucleus of the stria terminalis, in the medial amygdala and in the brainstem. Immunoreactive axons differed markedly in size and shape. Several pathways other than the hypothalamoneurohypophysial tract could be tentatively identified coursing, toward limbic structures, toward midline thalamic nuclei and toward the mesencephalon and brainstem. In contradistinction to what was observed in the rat in previous studies, no difference in the density of the innervation of the lateral septum was apparent when comparing male and female guinea pigs. A sex difference was, however, detected in the brainstem, where numerous immunoreactive cell bodies and axons were observed in the female but not in the male.