1. Electrophysiological recordings from ureteric mechanosensitive afferent fibres were performed using an in vitro preparation of the guinea-pig ureter and associated nerves. Single-unit recordings were obtained from small ureteric nerves arising from the inferior mesenteric ganglion, the hypogastric nerve or the pelvic plexus. The fibre composition of these ureteric nerves was also examined by electron microscopy. 2. In two ureteric nerve bundles, which were taken as representative of the maximal and minimal size of nerves used in the electrophysiological recordings, the number of nerve fibres was found to be 417 and 48, respectively. In the bigger nerve 12% of the fibres were small myelinated and the rest unmyelinated. The smaller nerve consisted of unmyelinated fibres only. 3. Electrophysiological recordings were made from sixty-seven mechanosensitive afferent fibres. The conduction velocities (CV) of forty-two of them were determined and all were found to be in the C fibre range (mean CV, 0.4 m/s). Of 119 additional fibres which were not further characterized 112 were C fibres (mean CV, 0.51 m/s) and seven were A delta fibres (mean CV, 3.78 m/s). 4. Mechanosensitive units were classified into two groups according to their ability to respond to contractions of the ureter: (i) U-1 units (9% of all mechanosensitive units) responded to contractions of the ureter and did not show on-going activity or after-discharges to mechanical stimulation. They had low thresholds to intraluminal distension (mean, 8 mmHg) and responded with a short latency to pressure stimuli. (ii) U-2 units (91% of all mechanosensitive units) did not respond to contractions of the ureter, had spontaneous activity between 0 and 2.4 Hz and exhibited after-discharges to mechanical stimuli lasting up to several minutes. They responded after a long latency (greater than 3 s) to distensions in the range of 5-30 mmHg. 5. The level of spontaneous activity and the pressure thresholds of the U-2 units were found to be different depending on whether or not the ureter was perfused intraluminally. Thus U-2 units recorded with intraluminal perfusion had a lower rate of on-going activity and higher threshold to intraluminal distension than U-2 units recorded without intraluminal perfusion. 6. Movement of an intraluminal glass bead under the receptive field of the units evoked strong responses in nine of eleven U-2 units tested as soon as their receptive fields were reached. 7. Our results demonstrate the existence of two classes of mechanosensitive afferent fibres in the guinea-pig ureter.(ABSTRACT TRUNCATED AT 400 WORDS)