Recent studies indicate that cholecystokinin (CCK) and serotonin (5-hydroxytryptamine, 5-HT) act via vagal afferent fibers to mediate gastrointestinal functions. In the present study, we characterized the interaction between CCK and 5-HT in the vagal primary afferent neurons. Single neuronal discharges of vagal primary afferent neurons innervating the duodenum were recorded from rat nodose ganglia. Two groups of nodose ganglia neurons were identified: group A neurons responded to intra-arterial injection of low doses of CCK-8 (10-60 pmol); group B neurons responded only to high doses of CCK-8 (120-240 pmol), and were also activated by duodenal distention. CCK-JMV-180, an agonist in high-affinity states and as an antagonist in low-affinity states, dose dependently stimulated group A neurons, but inhibited the effect of the high doses of CCK-8 on group B neurons. Duodenal perfusion of 5-HT evoked dose-dependent increases in nodose neuronal discharges. Some neurons that responded to 5-HT showed no response to either high or low doses of CCK-8. A separate group of nodose neurons that possessed high-affinity CCK-A receptors also responded to luminal infusion of 5-HT. Further, a subthreshold dose of CCK-8 (i.e., 5 pmol) produced no measurable electrophysiological effects but it augmented the neuronal responses to 5-HT. This potentiation effect of CCK-8 was eliminated by CR 1409. Conclusion: The vagal nodose ganglion contains neurons that may possess only high- or low-affinity CCK-A receptors or 5-HT3 receptors. Some neurons that express high-affinity CCK-A receptors also express 5-HT3 receptors. Pre-exposure to luminal 5-HT may augment the subsequent response to a subthreshold dose of CCK.