1. The afferent innervation of the distal ileum has been examined in normal rats and in rats treated at birth with capsaicin. Electrophysiological recordings were made using an in vitro preparation of distal ileum and its associated mesenteric nerves. The fibre composition of the mesenteric nerves was examined by electron microscopy and the numbers of primary afferent fibres innervating a segment of distal ileum was estimated using retrograde tracing. 2. Recordings were made from 120 single afferent units all of which showed some degree of background activity. The conduction velocities of sixty-seven afferent units were estimated, and all were found to be in the C-fibre range (less than 2 m/s). Eighty-two units were sufficiently studied to allow their classification according to whether they responded to mechanical stimuli (M units), chemical stimuli (Ch units) or both mechanical and chemical stimuli (MCh units). In control rats 85.5% were classified as MCh units, 11.9% as M units and 2.6% as Ch units. In capsaicin-treated rats six single and three multi-units were MCh and one multi-unit was classified as an M recording. 3. The effects of intraluminal distension were investigated in sixty-seven units which were classified according to whether or not they adapted during the distension. About half the total units were classified as rapidly adapting, the other half were slowly adapting. This distribution was similar for the MCh-units, but of the eight M units tested, seven adapted during distension. The distension thresholds were tested in thirty units, of which twenty-eight responded at thresholds below 18 mmHg. There were no differences in the thresholds of units from control and capsaicin-treated rats. 4. The chemosensitivity of units was tested in response to acetylcholine (ACh), bradykinin and substance P. Most units tested responded to ACh (78% of MCh units tested) and bradykinin (80% of MCh units), but fewer units responded to substance P (about 50% of MCh units). ACh produced an increased tension which outlasted the increase in afferent activity. Bradykinin gave long-lasting afferent responses which were not always accompanied by increases in tension. The increases in afferent activity produced by substance P were often seen after an increase in tension. 5. The fluorescent dye True Blue injected into the wall of the ileum labelled cell bodies in the spinal and nodose ganglia, predominantly on the left side of an animal. The mean number of labelled cells per animal was eighty-seven, of which the majority was in the T10-T13 spinal ganglia.(ABSTRACT TRUNCATED AT 400 WORDS)