The disulfide reducing activities of GSSG-and CoASSG-reductases were measured on partially purified extracts from a variety of prokaryotes and eukaryotes. Glutathione-reductase was found in varying amounts in all eukaryotes and prokaryotes, used in this study, with the exception of the two strict anaerobes Clostridium tartarivorum and Desulfovibrio vulgaris, and the two primitive Archaebacteria Methanosarcina barkeri and Halobacterium halobium. CoASSG-reductase was found in some eukaryotes and prokaryotes, but showed no clear pattern of distribution other than its absence whenever GSSG-reductase was not present. The absence of GSSG-reductase activity in organisms lacking GSH, confirms that glutathione metabolism is not universal and suggests that this enzyme might be useful as a marker in classifying organisms. The data suggest that glutathione-reductase occurs as a result of the change from a reducing to a oxidizing atmosphere in the primitive Earth.