AbstractSanguinarine, an isoquinoline alkaloid belonging to the benzophenanthridine class, has been found to be a time‐dependent inhibitor of cardiac Na,K‐ATPase activity. Enzyme inhibition depended on the concentration of enzyme in the assay. Dilution and centrifugation of the enzyme following incubation with sangunarine did not decrease inhibiton. Inhibiton of ouabain binding by sanguinarine was also time‐dependent and resulted in a reduction of the equilibrium level of ouabian bound to the enzyme preparation. This suggests the possibility that the positive inotropic effect of sanguinarine on the heart may be due to interaction with the cardiac glycoside receptor site on Na,K‐ATPase. Alternatively, sanguinarine may bind nonspecifically to phospholipids associated with the enzyme and exert its effects via conformational