1. The mechanism of action of oxytocin on vagal neurones of the rat was studied using single-electrode voltage-clamp recordings from brainstem slices. The ionic basis of the oxytocin-induced current was examined by changing the composition of the perfusion solution and by making use of channel blockers. 2. In neurones clamped at or near their resting potential, oxytocin generated a sustained, TTX-insensitive inward current whose peak amplitude was concentration related. This current was detectable at 10 nM, was half-maximal at about 100 nM and was maximal at micromolar concentrations of peptide. 3. The oxytocin current was inward over membrane potentials ranging from -110 to -20 mV and was voltage dependent, since it increased in magnitude as the membrane was depolarized from the resting potential toward less negative potentials. 4. Partial replacement of extracellular sodium by equimolar N-methyl-D-glucamine reversibly attenuated or suppressed the oxytocin current. By contrast, substituting part of extracellular chloride or blocking calcium currents did not modify it. Increasing the transmembrane potassium gradient was also without effect and none of the potassium channel blockers TEA, 4-amino pyridine (4-AP), apamin, caesium or barium affected the oxytocin current. This current is thus at least in part carried by sodium. 5. The activation of the oxytocin current as a function of the membrane potential could be quantitatively simulated using a Boltzmann equation, suggesting that oxytocin acts by inducing the opening of a voltage-dependent channel which can exist in either of two states, open or closed. 6. Lowering the extracellular calcium concentration from 2 to 0.1 mM, while keeping the magnesium concentration constant at 1 mM, enhanced the response to oxytocin. This low calcium-induced potentiation of the oxytocin current was 1.4-3-fold and was reversible. 7. We conclude that oxytocin increases the excitability of vagal neurones by generating a persistent, voltage-gated current which is sodium dependent, is insensitive to TTX and is modulated by divalent cations.