Transient potassium currents in mammalian central neurons influence both the repolarization of single action potentials and the timing of repetitive action potential generation. How these currents are integrated into neuronal function will depend on their specific properties: channel availability at the resting potential, activation threshold, inactivation rate, and current density. We here report on the voltage-gated transient potassium currents in embryonic mouse hippocampal neurons dissected at embryonic days 15–16 and grown in dissociated cell culture for up to 3 d. Two transient potassium currents, A-current and D-current, were isolated based on steady state inactivation and sensitivity to 4-aminopyridine (4-AP) and dendrotoxin (DTx). A-current had an activation threshold of approximately -50 mV and was half-inactivated at approximately -81 mV. A-current relaxations at voltages between -40 and +40 mV could be fit by single exponential functions with time constants of 20–25 msec; these time constants showed little sensitivity to voltage. In contrast, D-current had an activation threshold of between -40 and -30 mV and was half-inactivated at approximately -22 mV. D-current inactivation was voltage dependent; time constants of fitted exponential functions ranged from approximately 7 sec at -40 mV to 200 msec at +40 mV. A slower component of inactivation was also evident. D-current was preferentially blocked by 4-AP (100 microM) and DTx (1 microM). Operationally, A- and D- currents could be cleanly separated based on conditioning pulse potential and 4-AP sensitivity. Total transient potassium current amplitude increased during the time that neurons were in culture (recordings were made between 2 hr after dissociation and 3 d in culture). When normalized for cell capacitance (an index of membrane area), A-current density (pA/pF) decreased and D-current density increased, even during a period between days 1 and 3 when total transient current density remained constant. This observation suggests that A- and D-currents may be reciprocally modulated. Since blockade of D-current (with 100 microM 4-AP) increased both action potential duration and repetitive firing in response to constant current stimulation, long-term modulation of the A-current:D-current ratio may affect the excitability of hippocampal neurons.