Cholecystokinin and serotonin are released from the gastrointestinal tract in response to the products of digestion and play critical roles in mediating pancreatic secretion via vago-vagal reflex pathways. This study was designed to investigate the effects of activation of cholecystokinin CCK(1) and serotonin (5-hydroxytryptamine, 5-HT) 5-HT(3) receptors on pancreatic vagal afferent discharge and to determine whether there is an interaction between these receptors. Male Sprague Dawley rats anaesthetised with isoflurane (1.5%/100% O(2)) were used in all experiments. The effects of systemic administration of cholecystokinin and the serotonin 5-HT(3) receptor agonist phenylbiguanide on pancreatic vagal afferent discharge were recorded before and after administration of cholecystokinin CCK(1) and serotonin 5-HT(3) receptor antagonists. Cholecystokinin (0.1-10 microg/kg, i.v.) and phenylbiguanide (1 and 10 microg/kg, i.v.) increased pancreatic vagal afferent discharge dose-dependently. Cholecystokinin CCK(1) receptor antagonists, lorglumide (10 mg/kg, i.v.) and devazepide (0.5 mg/kg, i.v.), reduced cholecystokinin- and phenylbiguanide-induced increases in pancreatic vagal afferent discharge significantly (n=5, P<0.05). On the other hand, serotonin 5-HT(3) receptor blockade with granisetron (1 mg/kg, i.v.) or MDL72222 ([(1S,5R)-8-methyl-8-azabicyclo[3.2.1]octan-3-yl] 3,5-dichlorobenzoate; 0.1 mg/kg, i.v.) inhibited the pancreatic vagal afferent discharge responses to phenylbiguanide but not those to cholecystokinin. This study has confirmed that cholecystokinin and phenylbiguanide activate pancreatic vagal afferent discharge via activation of cholecystokinin CCK(1) and serotonin 5-HT(3) receptors, respectively. In addition, it has demonstrated that (i) the serotonin 5-HT(3) agonist phenylbiguanide acts partly via an interaction with cholecystokinin CCK(1) receptors, and (ii) the actions of cholecystokinin are not dependent on serotonin 5-HT(3) receptor activation.