Abstract. Sanguinarine, an alkaloid from Sanguinaria canadensis, has no effect on the yeast Saccharomyces cerevisiae at concentrations of up to 225M. Yeast cells become sensitive to sanguinarine and lose cytosolic K+ in a time- and concentration-dependent manner when they express the mammalian Na+,K+-ATPase (sodium pump). Dose-response studies show that sanguinarine induces K+ outflow from cells expressing wild-type sodium pumps with an EC50 of 29.31.2M. A similar effect with a comparable EC50 of 26.81.3M is obtained with cells expressing an Asp369Ala mutant of the sodium pump 1 subunit. Since this sodium pump mutant does not hydrolyze ATP, it can be excluded that the observed sanguinarine-induced outflow of K+ is an active ion transport process. Ouabain inhibits the sanguinarine effect at concentrations higher than 1mM. In contrast, proscillaridin A inhibits the sanguinarine-induced K+ outflow from cells expressing the wild-type sodium pump with an IC50 of 48.91.3M. A similar IC50 of 52.23.0M is obtained with cells expressing the Asp369Ala mutant. These data, together with the fact that sanguinarine inhibits the binding of [3H]ouabain to microsomes prepared from yeast cells expressing the sodium pump with an IC50 of 94.54.3M, all indicate that sanguinarine specifically targets the sodium pump, and that the observed K+ outflow is tightly associated with the presence of the enzyme.