1. The effects of vasoactive intestinal polypeptide (VIP) on isolated parasympathetic neurones of rat intracardiac ganglia were examined under voltage clamp using dialysed and perforated patch whole-cell and excised outside-out membrane patch recording configurations. 2. VIP reversibly potentiated nicotinic ACh-evoked whole-cell currents, with half-maximal potentiation (EC50) obtained with 260 pM VIP. However, VIP had no effect on muscarinic ACh-evoked currents, ATP-evoked currents, or depolarization-activated ionic currents in these neurones. 3. VIP-induced potentiation of nicotinic ACh-evoked whole-cell currents was observed following cell dialysis, and was inhibited reversibly by bath application of the VIP receptor-binding inhibitor L-8-K (5 microM) or the neuronal nicotinic receptor antagonist mecamylamine (3 microM). 4. The signal transduction pathway mediating VIP-induced potentiation of nicotinic ACh-evoked currents involves a guanine nucleotide-binding protein (G-protein) but not cyclic AMP. Intracellular application of 100 microM GDP-beta-S, or pre-incubation of neurones with pertussis toxin, inhibited VIP-induced potentiation of ACh-evoked whole-cell currents. 5. In outside-out membrane patches, co-application of ACh (4 microM) and VIP (4 nM) decreased the duration of closings between bursts and clusters of bursts of ACh single-channel activity relative to control (4 microM, ACh alone). VIP, however, did not alter single ACh receptor channel current amplitude, duration of closings and openings within a burst, or mean burst duration. 6. VIP-induced modification of nicotinic ACh receptor channel kinetics results in an increase in the open-channel probability which is sufficient to account for the VIP-mediated potentiation of nicotinic ACh-evoked whole-cell currents. 7. The potentiation of nicotinic ACh-evoked currents by VIP is likely to account for the altered neuronal activity observed in the mammalian intracardiac ganglia in vivo and consequent changes in heart rate and cardiac contractility.