Sites which bind oxytocin and vasopressin with high affinity were detected in the brain and upper spinal cord of 12 human subjects, using in vitro light microscopic autoradiography. Tissue sections were incubated with tritiated vasopressin, tritiated oxytocin or an iodinated oxytocin antagonist. The ligand specificity of binding was assessed with unlabelled vasopressin or oxytocin in excess, as well as in competition experiments using synthetic structural analogues. The distribution of vasopressin binding sites differed markedly from that of oxytocin binding sites in the forebrain, while there was overlap in the brainstem. Vasopressin binding sites were detected in the dorsal part of the lateral septal nucleus, in midline nuclei and adjacent intralaminar nuclei of the thalamus, in the hilus of the dentate gyrus, the dorsolateral part of the basal amygdaloid nucleus and the brainstem. The distribution of oxytocin binding sites in the brainstem has been recently reported (Loup et al., 1989). Oxytocin binding sites were also observed in the basal nucleus of Meynert, the nucleus of the vertical limb of the diagonal band of Broca, the ventral part of the lateral septal nucleus, the preoptic/anterior hypothalamic area, the posterior hypothalamic area, and variably in the globus pallidus and ventral pallidum. The presence of oxytocin and vasopressin binding sites in limbic and autonomic areas suggests a neurotransmitter or neuromodulator role for these peptides in the human central nervous system. They may also affect cholinergic transmission in the basal forebrain and consequently play a role in Alzheimer's disease.