Using the whole-cell patch-clamp technique, at least three types of voltage-gated currents expressed by cultured rat microglia were identified: an inward rectifier K+ current, a delayed rectifier K+ current (IK), and a Na+ current activated by depolarization. The inward rectifier conductance was activated by hyperpolarization to potentials more negative than -80 mV, depended on the external K+ concentration, and declined over time during whole cell recording, as the cell was internally dialyzed. The delayed rectifier current was activated by depolarization to potentials more positive than -40 mV and the rates of activation and deactivation showed a voltage-dependence similar to such currents seen in other preparations. An inward current possibly carried by Na+ was seen in a small percentage of cells. Recordings had been made from two morphological cell types, namely process-bearing ("ramified") and non-process-bearing ("ameboid"). Each of these currents was present in microglia of both morphological types. However, microglial morphology, which is thought to represent different states of activation, was significantly related to the types of combinations of currents expressed in a given cell.