The existence of vasopressin-sensitive neurons in the nucleus of the solitary tract of the rat and the presence in this brain area of vasopressin binding sites were investigated using extracellular single-unit recordings from brain-stem slices and light microscopic autoradiography. About 45% of the recorded neurons responded to vasopressin at 5-2000 nM by a reversible, concentration-dependent increase in firing rate. The action of vasopressin was direct, was suppressed by a vasopressor antagonist, and was mimicked by a vasopressor agonist. Oxytocin was 10-100 times less efficient than vasopressin and a specific antidiuretic agonist was without effect. Using light microscopic autoradiography and 3H-arginine vasopressin as a ligand, high-affinity vasopressin binding sites were found distributed over the whole rostrocaudal extent of the nucleus of the solitary tract. Binding was displaced by unlabeled vasopressor agonist but not by unlabeled antidiuretic agonist. Thus, the nucleus of the solitary tract contains V1-type vasopressin receptors which are, at least in part, located on neuronal membranes and whose activation generates bioelectrical signals. Solitary tract vasopressin-sensitive neurons may be the target of a vasopressinergic innervation originating in the hypothalamic paraventricular nucleus and could be involved in the central regulation of cardiovascular functions.