In slices from the rat brain, extracellular recordings were obtained from single neurones located in the lateral septum, an area known to receive a vasopressinergic innervation. Approximately half of the neurones tested responded to vasopressin by a concentration-dependent increase in firing rate, the lowest effective concentration being in the order of 2 nM. The effect of vasopressin was blocked by a synthetic structural analogue possessing vasopressor and oxytocic antagonistic properties on peripheral vasopressin and oxytocin receptors. Oxytocin had a weak effect in firing septal neurones, whereas a selective oxytocic agonist was totally ineffective. The action of vasopressin on neuronal firing was mimicked by a vasopressor agonist (Phe2-Orn8-VT) but not by a selective antidiuretic agonist (dDAVP). These results indicate that the vasopressin receptors present in rat septum are V1 (vasopressor type) rather than V2 (antidiuretic type) receptors. In addition, we conclude that these receptors, when occupied, lead to increased firing of lateral septal neurones.