The nature of the putative transmitter(s) mediating the non-cholinergic excitatory post-synaptic potential (e.p.s.p.) described in the preceding paper was investigated by means of electrophysiological, pharmacological and immunohistochemical methods. Serotonin (1-10 microM) when applied by superfusion caused a slow depolarization that closely mimicked the synaptic response in about 60% of the coeliac neurones that exhibited a non-cholinergic e.p.s.p. The serotonin depolarization evoked in low-Ca2+, high-Mg2+ solution or in a Krebs solution containing cholinergic antagonists was quantitatively similar to that elicited in normal Krebs solution. When compared in the same neurones the membrane resistance change during the course of the serotonin depolarization and of the non-cholinergic e.p.s.p., as well as their respective responses to conditioning polarization, were similar. The non-cholinergic e.p.s.p. was reversibly abolished during serotonin-induced depolarization; the blockade persisted when the membrane potential was restored to the resting level by hyperpolarizing current. The serotonin depolarization as well as the non-cholinergic e.p.s.p. were reversibly suppressed by cyproheptadine (20-50 microM), a serotonin antagonist, and enhanced by fluoxetine (30-50 microM), a serotonin reuptake inhibitor. On the other hand, pre-treating the ganglia with L-tryptophan (50 microM), a precursor of serotonin, preferentially augmented the synaptically induced response. A portion of the neurones (15%) were depolarized by substance P (1 microM) which also reversibly desensitized the non-cholinergic e.p.s.p. elicited in these neurones. The remaining neurones (25%) were insensitive to either serotonin or substance P, and the non-cholinergic e.p.s.p.s elicited in these cells were likewise not appreciably affected by these two agents. Furthermore, cyproheptadine, fluoxetine and L-tryptophan had no significant effect on the non-cholinergic e.p.s.p.s elicited in serotonin-insensitive neurones. Using the immunohistofluorescent techniques, dense but unevenly distributed serotonin immunoreactive nerve fibres could be observed surrounding many coeliac neurones. Immunoreactivity was not observed in the ganglia incubated with antisera pre-absorbed with excess serotonin. Collectively our results suggest that serotonin is the mediator of non-cholinergic e.p.s.p.s. elicited in about 60% of coeliac neurones sampled in this study, and that in the remaining neurones the slow depolarization may be generated by substance P and/or some unknown transmitter(s).