Sympathetic neurons, dissociated from neonatal rat superior cervical ganglia, were voltage clamped with two microelectrodes. Depolarization from resting potential activated a rapid transient inward current carried by sodium and a slow inward current blocked by cobalt. Depolarization from resting potential also activated up to three kinetically distinct outward currents, which were further studied by tail current analysis. Following long depolarizing steps, outward current decayed biphasically. The fast phase (delayed rectifier) decayed over 10-20 ms. The slow phase (calcium dependent) required as much as 1-2 s to decay to base line. A small component of the total outward current was a persistent current activated between -70 and -30 mV (M-current), which decayed over 200-300 ms. This current was studied in isolation following hyperpolarizing steps from potentials negative to the threshold for activation of the other delayed outward currents. Tetraethylammonium (TEA) blocked the fast tail current, partially inhibited the slow tail current, and reduced M-currents. Cobalt selectively decreased the slow tail current. Muscarine blocked M-current but not other outward currents. A transient outward current was activated by depolarization from only holding potentials negative to -60 mV. This current peaked in 10-20 ms and decayed over about 50 ms. A persistent ("anomalous") inward current was evoked by hyperpolarizing steps from only holding potentials negative to -50 to -60 mV. These seven membrane currents may be separately characterized on the basis of their voltage- and time-dependent properties. Further identification is aided by the use of channel-blocking chemicals, although the latter may lack specificity, especially when used to study potassium channels.