Blood transfusion is the second most important mechanism of transmission of Chagas' disease. Gentian violet, a cationic dye, is currently used in blood banks in endemic areas in attempts to eliminate such transmission. A photodynamic action of gentian violet has been demonstrated in Trypanosoma cruzi. Visible light causes photoreduction of gentian violet to a carbon-centered radical. Under aerobic conditions this free radical autooxidizes generating superoxide anion whose dismutation yields hydrogen peroxide. This photodynamic action of gentian violet is thus probably mediated by the oxygen reduction products. Since irradiation with visible light in the presence of sodium ascorbate reduces the effective dose and time of contact of the dye with T. cruzi-infected blood, a possible application of these findings can be envisaged. In addition to this photodynamic action, an uncoupling effect of gentian violet on mitochondrial oxidative phosphorylation has been described in rat liver and T. cruzi mitochondria. Gentian violet released respiratory control, hindered ATP synthesis, enhanced ATPase activity, released the inhibition of State 3 respiration by oligomycin, and produced swelling of isolated rat liver mitochondria or T. cruzi mitochondria in situ. Taken together, these results indicate that the T. cruzi mitochondrion is the main target of gentian violet toxicity in the dark.