The electrophysiological action of arginine vasopressin on neurones in the lateral septum of the rat brain was studied using extracellular recordings and the in vitro brain slice technique. Of 177 neurones tested in the presence of vasopressin at 1-1000 nM, 77 (about 44%) responded by a reversible increase in firing rate, 12 (about 7%) were inhibited and the remaining were not affected. The lowest peptide concentration effective in exciting septal neurones ranged between 1 and 50 nM, and the magnitude of the excitatory effect was concentration dependent. At high vasopressin concentrations, the peptide-induced excitation was often followed by a transient pause in firing; this was probably due to action potential inactivation, brought about by the vasopressin-induced neuronal membrane depolarization. The excitatory effect of vasopressin was postsynaptic, since it was not abolished following synaptic blockade in a low calcium-high magnesium perifusion solution. A comparison of the effects of vasopressin and oxytocin suggested that most of the septal vasopressin-sensitive neurones are endowed with vasopressin receptors, whereas a minority of them bear oxytocin receptors.