1. Electrophysiological properties of acutely dissociated neurons from the major pelvic ganglion (MPG) of the adult male rat were studied with whole cell patch-clamp recording techniques. The MPG neurons innervating the urinary bladder were labeled by retrograde axonal tracing methods with the use of a fluorescent dye, Fast Blue (FB) injected into the bladder wall and identified with a fluorescent microscope. 2. Passive and active membrane properties such as resting membrane potential, input resistance, duration of action potentials, thresholds for spike activation, or duration of afterhyperpolarization in unidentified MPG neurons were comparable with those of FB-labeled neurons innervating the urinary bladder. The action potential in both unidentified and bladder efferent MPG neurons was reversibly abolished by tetrodotoxin (TTX, 1 microM). The afterhyperpolarization of the TTX-sensitive action potential in both groups was reduced by application of Cd2+ (0.1 mM) and further suppressed by tetraethylammonium (TEA, 10 mM). Extracellularly applied TEA increased the duration of the action potential, and 4-aminopyridine (4-AP, 1 or 2 mM) also reduced the spike afterhyperpolarization and increased the spike duration. The duration of the action potential was decreased and the rate of spike repolarization was increased by approximately 2.5-fold with negative shift of membrane potential from -40 to -80 mV. 3. The isolated Na+ current was reversibly blocked by 1 microM TTX and had a mean peak amplitude of 127.3 pA/pF when activated from a holding potential of -70 mV in the external solution containing 100 mM Na+. The Na+ conductance reached half-maximal activation at a membrane potential of -21.5 mV with a slope factor of 4.9 mV. The steady-state inactivation of Na+ conductance occurred at membrane potentials more depolarized than -90 mV, and the half-maximal inactivation was obtained at -57.5 mV with a slope factor of 8.8 mV. 4. The fast-transient A-type K+ current (IA) was activated at membrane potentials more depolarized than -60 mV from a holding membrane potential of -100 mV, reached a peak amplitude within 10 ms after the onset of depolarizing voltage steps, and decayed within 20-30 ms at membrane potential depolarizations to +20 to +30 mV. The IA current activated by a voltage step to +20 mV from a holding potential of -100 mV averaged 102.1 pA/pF. The half-maximal activation of the IA conductance was obtained at a membrane potential of -21.2 mV with a slope factor of 9.9 mV. In steady-state inactivation of IA current, the half-maximal inactivation occurred at -76.5 mV and the slope factor was 8.0 mV. 5. The delayed K+ current was reduced by 25-35% by bath application of Cd2+ or the elimination of extracellular Ca2+ ions. The bath application of 4-AP (2 mM) suppressed the IA current by 75% and the delayed K+ current by 60%. Extracellularly applied TEA (10 mM) suppressed the delayed K+ current by 90%, but suppressed the IA current by only 16%. 6. These results indicate that bladder neurons and unidentified neurons in the MPG have similar properties including a TTX-sensitive Na+ current and three distinct types of voltage-sensitive K+ currents-IA current, Ca(2+)-activating K+ current, and delayed rectifier K+ current-that contribute to the repolarization phase of the action potential. These electrical properties of the MPG neurons resemble those of sympathetic neurons in the superior cervical and inferior mesenteric ganglia.