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Na + , K + -ATPase of the plasma membrane isolated from sheep kidney medulla exhibits functio­nal asymmetry for the cardiac glycoside ouabain. In this vesicular membrane preparation the rate of binding of ouabain was slow (time constant &gt; 60 min) when the vesicles were incubated in the presence of isotonic sucrose. Upon treatment of the preparation with hypoosmotic shock or phos­pholipase A the initial rate of ouabain binding was enhanced at least 3 fold. In equilibrium a concentration of the ouabain-enzyme-complex was obtained which was about twofold that of the untreated vesicles. This result suggests two types of ouabain binding sites with an approximate stoichiometry of 1 to 1. The stoichiometry seems to be maintained at high concentrations of ouabain where binding curves show a biphasic time course. Additional information about hetero­geneity of binding sites comes through experiments in which the vesicles were treated with Mg 2+  prior to the addition of ouabain. A minor fraction of the binding sites were occupied by ouabain only after longtime incubation with Mg 2+ .