A putative 5-HT4 receptor-mediated depolarization of the rat isolated vagus nerve has been studied using a grease-gap extracellular recording technique. Ondansetron (1 μM) was used to block the predominant 5-HT3 receptor mediated depolarization in this preparation and the effects of the 5-HT4 receptor antagonists DAU 6285 (endo-8-methyl-8-azabicyclo [3.2.1] oct-3-y1-2,3-dihydro-6-methoxy-2-oxo-1H-benzimidazole-1-carboxylate HCl); 0.3, 1.0 or 3.0 μM and SDZ 205–557 (2-methoxy-4-amino-5-chloro-benzoic acid 2-(diethylamino)-ethyl ester HCl); 0.1, 0.3 or 1.0 μM were studied on the residual, ondansetron-resistant, component of the response. The effects of the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) and of forskolin on the ondansetron-resistant response were also studied. Both DAU 6285 and SDZ 205–557 acted as competitive antagonists of the ondansetron-resistant response to 5-HT with pA2 values of 6.8 (6.7–7.1, n = 12) and 7.1 (6.9–7.5, n = 12) respectively. The vagus nerve was depolarized by IBMX (100 μM) or forskolin (10 μM), the effects being similar to the maximum response to 5-HT. In the presence of IBMX (100 μM) or forskolin (10 μM) the ondansetron-resistant component of the response to 5-HT was enhanced and the 5-HT3 receptor-mediated component reduced. These results with DAU 6285 and SDZ 205-557 are consistent with a 5-HT4 receptor-mediated mechanism of the ondansetron-resistant depolarizing response to 5-HT.