Microglia, the resident macrophages of the brain, are transformed from a quiescent into an activated phenotype in a number of pathological conditions. The signalling mechanisms which control such transformations are not yet understood. In the present study, we have characterized fast electrophysiological responses in cultured microglia, induced by two putative signalling substances, complement 5a (C5a), a chemotactic agent for macrophages and microglia, and epidermal growth factor, the receptor of which is up-regulated during pathological conditions in the brain. Both factors transiently activate an outwardly rectifying K+ conductance, while the membrane of the unstimulated microglial cell is dominated by an inwardly rectifying K+ conductance. The C5a-stimulated current developed within about 20s and decayed within a slightly slower time course. It was activated by depolarlizing voltage steps positive to the resting membrane potential of about -70 mV, and neither inactivated nor showed a delayed activation following voltage steps. The epidermal growth factor-stimulated current showed similar characteristics. When G-proteins were specifically blocked, the K+ conductance could no longer be activated by C5a or epidermal growth factor, suggesting that for both agonists an inhibitory G-protein is involved in the intracellular signalling cascade. We tested if the induction of the K+ conductance is causally linked to other C5a-induced cellular responses, like transient cytosolic Ca2+ elevation and mobility. The K+ conductance was not activated when a Ca2+ transient was induced by thapsigargin, nor did a blockade of the C5a-induced K+ conductance by K+ channel blockers affect the motility response. This implies that after activation of the C5a receptor and the G-protein, the K+ conductance activation, the Ca2+ mobilization and the motility response are governed by independent intracellular pathways, and that the K+ conductance increase must serve other functions than the control of motility.