NOTE: THE CHARACTERS (SbIII) IN THIS ABSTRACT CANNOT BE DISPLAYED CORRECTLY ON THIS PAGE. PLEASE REFER TO THE ABSTRACT ON THE PUBLISHER’S WEBSITE FOR AN ACCURATE DISPLAY. Despite extensive use of antimonial compounds in the treatment of leishmaniasis, their mode of action remains uncertain. Here we show that trivalent antimony (SbIII) interferes with trypanothione metabolism in drug-sensitive Leishmania parasites by two inherently distinct mechanisms. First, SbIII decreases thiol buffering capacity by inducing rapid efflux of intracellular trypanothione and glutathione in approximately equimolar amounts. Second, SbIII inhibits trypanothione reductase in intact cells resulting in accumulation of the disulfide forms of trypanothione and glutathione. These two mechanisms combine to profoundly compromise the thiol redox potential in both amastigote and promastigote stages of the life cycle. Furthermore, we demonstrate that sodium stibogluconate, a pentavalent antimonial used clinically for the treatment for leishmaniasis, induces similar effects on thiol redox metabolism in axenically cultured amastigotes. These observations suggest ways in which current antimony therapies could be improved, overcoming the growing problem of antimony resistance.