The exposure of phosphatidylserine (PS) at the cell surface plays a critical role in blood coagulation and serves as a macrophage recognition moiety for the engulfment of apoptotic cells. Previous observations have shown that a high extracellular [K(+)] and selective K(+) channel blockers inhibit PS exposure in platelets and erythrocytes. Here we show that the rate of PS exposure in erythrocytes decreases by approximately 50% when the intracellular [K(+)] increases from 0 to physiological concentrations. Using resealed erythrocyte membranes, we further show that lipid scrambling is inducible by raising the intracellular [Ca(2+)] and that K(+) ions have a direct inhibitory effect on this process. Lipid scrambling in resealed ghosts occurs in the absence of cell shrinkage and microvesicle formation, processes that are generally attributed to Ca(2+)-induced lipid scrambling in intact erythrocytes. Thus, opening of Ca(2+)-sensitive K(+) channels causes loss of intracellular K(+) that results in reduced intrinsic inhibitory effect of these ions on scramblase activity.