Microglia were isolated from a murine neonatal brain cell culture in which their development had been stimulated by supplementation with the macrophage/microglial growth factor macrophage colony-stimulating factor (M-CSF). Using the wholecell configuration of the patch-clamp technique, voltage-gated membrane currents were recorded from these microglial cells. Hyperpolarization induced inward rectifying K ยง currents, as described for microglia from untreated cultures. These currents activated negative to the K + equilibrium potential and, with a strong hyperpolarization, displayed time-dependent inactivation. The inactivation was abolished when extracellular NaC1 was replaced by N-methyl-D-glucamine (NMG), thereby indicating a partial block of this K ยง conductance by Na ยง Inward rectifying currents were also blocked by extracellularly applied Cs + or Ba 2+. They were slightly diminished following treatment with extracellular tetraethylammonium chloride (TEA) but were not affected by 4-aminopyridine (4-AP). Upon long lasting depolarizing voltage pulses to potentials positive to 0 mV, the cells exhibited a slowly activating H ยง current which could be reduced by application of inorganic polyvalent cations (Ba 2 ยง Cd 2 ยง Co 2 ยง La 3+, Ni 2 ยง Zn 2+) as well as by 4-AP or TEA. Based on their kinetics and pharmacological characteristics, both currents detected on M-CSF-grown microglia are suggested to correspond to the inward rectifier and the H ยง