1. Whole-cell patch-clamp recording in combination with axonal tracing techniques was used to examine the electrical properties of afferent neurones innervating the urinary bladder of the adult rat. Individual bladder afferent cells were labelled by Fast Blue (FB), injected into the bladder wall. 2. Passive and active electrical parameters at room temperature (20-22 degrees C) in FB-labelled bladder afferent neurones were comparable with those in unlabelled neurones. Unselected dorsal root ganglion (DRG) neurones as well as bladder afferent neurones exhibited two different types of action potential: high-threshold humped spikes in small-sized neurones and low-threshold narrow spikes in large-sized neurones. 3. The majority (70%) of bladder neurones which were small in size expressed high-threshold tetrodotoxin (TTX)-resistant Na+ channels and slow-inactivating A-type K+ channels (KA), which were available at the resting membrane potential, whereas large-sized DRG neurones had low-threshold TTX-sensitive Na+ channels and fast-inactivating KA channels, which were almost completely inactivated at the resting membrane potential. 4. Half-maximal conductances of activation of TTX-resistant and TTX-sensitive Na+ currents were obtained at -10.3 and -25.3 mV, respectively. The TTX-resistant and TTX-sensitive Na+ currents were half-inactivated at -25.3 and -56 mV, respectively. 5. In the TTX-resistant neurones, the transient outward K+ current (A-type current, IA) with half-maximal conductance at -40.8 mV was half-inactivated at -77.5 mV, and exhibited slower decaying kinetics (mean decay constant (tau), 240 ms) than the IA current recorded from the large-sized TTX-sensitive neurones (mean tau, 20 ms). 6. These results suggest that the majority of bladder afferent neurones have high electrical thresholds for spike activation due to the TTX-resistant Na+ current and the slow-inactivating IA current, which reflect the large population of unmyelinated high-threshold C fibre afferents that innervate the urinary bladder.