Postganglionic parasympathetic neurons of the opossum gallbladder were studied using morphological and intracellular electrophysiological recording techniques. On average there were 17 ganglia/cm2 with 8 neurons/ganglion arranged as loosely or densely packed clusters. Intracellular injection of horseradish peroxidase identified two types of neurons. Most of the neurons (approximately 80%) had a single long process that extended beyond the ganglion border with few or no short additional neurites. The other type was multipolar, having several processes that did not appear to extend beyond the ganglion border. Gallbladder neurons had passive and active electrical properties similar to other parasympathetic ganglion cells. Most of neurons studied (71%) responded to intracellular injection of suprathreshold depolarizing current with a short burst of action potentials. The remaining neurons (29%) responded with only an initial action potential. Seventy percent of the neurons tested received nicotinic fast excitatory postsynaptic potentials (EPSPs). Nineteen percent of these neurons also exhibited a slow depolarizing response following repetitive orthodromic nerve stimulation. Cholecystokinin octapeptide sulfate (CCK-8, 0.1-500 nM) had no effect on resting membrane potential or membrane input resistance. However, CCK-8 increased the amplitude of fast EPSPs evoked by orthodromic nerve stimulation by facilitating release of acetylcholine because CCK-8 had no effect on the postsynaptic response to exogenously applied acetylcholine. These data suggest that the ganglionated plexus of the gallbladder may mediate some of the observed effects of CCK-8 on gallbladder motility.