D-current is a fast activating, slowly inactivating potassium current that is sensitive to alpha-dendrotoxin (a-DTX) and low concentrations of 4-aminopyridine (4-AP). In hippocampal pyramidal neurons, this current has been implicated in delayed excitation and temporal integration of subthreshold depolarizing inputs (Storm, Nature, 1988) and in control of calcium spikes and burst discharges (Golding et al, J.Neurosci.,1999). However the roles of D-current in action potential repolarization under various conditions remain to be elucidated. Here we have examined how D- current contributes to spike repolarization in a membrane potential-dependent manner. Whole-cell patch clamp recordings were obtained from CA1 pyramidal neurons in hippocampal slices from young adult male rats (3–5 weeks). Action potentials were evoked by injection of long- lasting (1.0 s) and brief (200 ms) current pulses of different intensities. Since the D- current inactivates between ­120 mV and ­60 mV, we varied the background membrane potential by DC current injection, and tested the effects of a-DTX at hyperpolarized (­80 mV) and depolarized (­60 mV) levels. At ­80 mV, application of 1 uM a-DTX caused an increase in the half width and decay time of the first spike, and shortened the characteristic D-current-dependent delay to the first action potential in response to long, weak pulses (Storm, 1988). In contrast, when holding the cell at ­60 mV (at which the D-current is largely inactivated), application of a-DTX had no detectable effect on the half width and decay time of the action potential. These data indicate that an a-DTX-sensitive current, the D- current, contributes to spike repolarization in CA1 hippocampal pyramidal cells in a voltage- dependent manner. [Supported by NFR/Forskerlinjen/STORFORSK.]