1. Intracellular, electrophysiological techniques were combined with radio-immunological, chromatographic and pharmacological techniques to determine if nerve terminals containing substance P mediated transient depolarizing responses of principal ganglion cells induced by neurotensin. Experiments were performed in vitro on guinea-pig inferior mesenteric ganglia. 2. In 61% of principal ganglion cells tested in normal ganglia, neurotensin caused a transient membrane depolarization. In ganglia which were removed from animals which had been pre-treated with capsaicin, transient responses to neurotensin were virtually abolished. 3. In normal ganglia, neurotensin increased the amplitude and duration of noncholinergic slow EPSPs evoked by electrical stimulation of the lumbar colonic nerve. Such increases were absent in ganglia obtained from animals pre-treated with capsaicin. 4. In guinea-pigs pre-treated with capsaicin, the content of substance P-like material was significantly reduced in inferior mesenteric and coeliac ganglia, dorsal root ganglia and lumbar spinal cord, compared to control animals. The content of substance P-like material in segments of distal colon was slightly reduced. The content of vasoactive intestinal polypeptide-, cholecystokinin- and bombesin-like material in the same tissues from animals pre-treated with capsaicin was not significantly different from control animals. 5. Chromatographic analysis using HPLC (high-performance liquid chromatography) techniques revealed that the material depleted from inferior mesenteric and coeliac ganglia, dorsal root ganglia and lumbar spinal cord by capsaicin pre-treatment co-eluted with synthetic substance P. 6. Electrical stimulation of the lumbar colonic nerve released substance P-like material from isolated inferior mesenteric ganglia as determined by radioimmunoassay of samples of superfusate. Exogenous administration of neurotensin caused a significant increase in the amount of substance P-like material released during nerve stimulation. 7. Transient depolarizing responses evoked by neurotensin were markedly attenuated when ganglion cells were postsynaptically desensitized to exogenously administered substance P. 8. Taken together, these findings suggest that transient depolarizations mediated by an indirect action of neurotensin and facilitation of electrically evoked non-cholinergic slow EPSPs by neurotensin involved presynaptic release of substance P from collateral nerve terminals of primary afferent nerve fibres in the inferior mesenteric ganglion. 9. It was suggested that under normal in vivo conditions, neurotensin or a C-terminal-related peptide contained in central preganglionic nerve endings might function as an excitatory neuromodulator to enhance the release of substance P from primary afferent nerve terminals thereby facilitating non-cholinergic peripheral afferent synaptic input to prevertebral ganglion cells.