A specific inhibitor of SERCA-pumps, thapsigargin (TG) was used to demonstrate the direct involvement of the SR Ca2+-ATPase in passive K+/Na + exchange. The K+-potential variations across vesicle membranes were measured in the absence of ATP with a fluorescent probe: 3,3'-dipropylthiodicarbocyanine iodide. Addition of EGTA dissipates the K÷-potential whereas the presence of TG abolishes this effect. Our data prove that the CaZ+-ATPase translocates monovalent cations at a rate similar to the E2-~Et conformational change.