Membrane mechanisms involved in the regulation of nerve cell excitability were investigated in sympathetic neurons of bullfrog using standard intracellular recording and the two electrode voltage clamp technique. Two fast outward currents have been identified and partially characterized. One is dependent on calcium for activation and inactivates very quickly at membrane voltages positive to resting potential. Since treatments which block this current also slow action potential repolarization, and since this current activates maximally in a few msec, we have proposed that this calcium-dependent fast outward current is responsible for action potential repolarization. The other fast outward current is not calcium dependent and inactivates over a range of membrane potentials negative to resting potential. This current is similar to a potassium current identified in other neurons called A-current. A-current can influence the likelihood of repetetive firing of a neuron. Studies on other potassium currents in sympathetic neurons have continued. Finally, changes is intracellular free calcium concentration were measured simultaneously with changes in membrane potential using the calcium sensitive dye, Arsenazo III.