Abstract Inhibition of yeast plasma membrane ATPase by vanadate occurs only if either Mg 2+ or MgATP 2− is bound to the enzyme. The dissociation constant of the complex of vanadate and inhibitory sites is 0.14–0.20 μM in the presence of optimal concentrations of Mg 2+ and of the order of 1 μM if the enzyme is saturated with MgATP 2−. The dissociation constants of Mg 2+ and MgATP 2− for the sites involved are 0.4 and 0.62–0.73 mM, respectively, at pH 7. KCl does not increase the affinity of vanadate to the inhibitory sites as was found with ( Na + + K +)- ATPase . On the other hand, the effect of Mg 2+ upon vanadate binding is similar to that upon ( Na + + K +)- ATPase , and the corresponding affinity constants of Mg 2+ and vanadate for the two enzymes are of the same order of magnitude.