Printed in Great Britain A transport model for the ( Na + / K + ) ATPase in

(Na+/K+)ATPase liposomes of various degrees of reconst i tu t ion are formed by varying the amount of phosphatidylcholine added to the soluble (Na+/K+) ATPase before vesicles are formed by cholate removal. In the presence of ATP) the reconstituted sodium pump effectuates (Na+/K +) antiport. In the absence of ATP, the reconstituted sodium pump forms a (Na+/K +) channel. The stable plateaus formed by ( l ) the active Na + transport, (2) the active K + transport) (3) the 'passive' Na + flux) and (4) the 'passive' K + flux are determined in the optimally and the partially reconst i tuted liposomes. The activities of all four vectorial functions vary in a tightly correlated fashion) suggesting that they are mediated by the same transport-active configuration of (Na+/K+)ATPase. A transport model which includes the active and the passive (Na+/K +) fluxes mediated by the sodium pump in liposomes is outlined.