Dithiothreitol (DTT) increased the generation of angiotensin when bovine renin acted on bovine substrate or hog substrate. Kinetic analysis indicated that the Vmax increased without a change in Km. Pretreatment of bovine renin with DTT followed by dilution of the DTT a low concentration did not augment renin activity, while pretreatment of the substrate and a similar dilution revealed marked potentiation. N-Ethylmaleimide (NEM) did not inhibit renin activity, but prevented the augmenting effect of DTT present during the incubation. DTT appeared to augment renin activity by an action on renin substrate. Renin probably does not fall in the category of sulfhydryl-dependent enzymes.