Microglial cells were cultured from murine neonatal brain. Ramification of isolated microglia could be induced by the application of astrocyte-conditioned medium (ACM). Voltage-gated outward potassium currents (IK) were measured in ramified microglial cells 12-24 h after their treatment with ACM. The effects of the specific K+ channel blockers charybdotoxin (CTX), noxiustoxin (NTX) and kaliotoxin (KTX) on IK of ramified microglia were studied. All these peptide toxins blocked IK in a concentration-dependent manner, while showing a high sensitivity for IK. A half-maximal effective concentration (IC50) of CTX was estimated to be 1.13 nM, while IC50 values of 1.24 nM and of 0.81 nM were calculated for KTX and for NTX, respectively. In contrast, dendrotoxin (DTX) did not show any effect on IK. It is suggested that ramified microglial cells express outward K+ currents exhibiting pharmacological properties similar to that of outward K+ currents in cytokine-activated ameboid microglia.