THE catheptic enzymes, which are concerned with proteolysis in animal tissue, undergo activation when treated with certain substances, such as HCN or H2S. These characteristic activation phenomena became of increased physiological interest with the discovery of the occurrence in tissues of natural activators, as for example, zookinase [Waldschmidt-Leitz et al., 1930], which has been shown by Waldschmidt-Leitz and Purr [1931] to be identical with reduced glutathione. The same activator has been described by Grassmann et al. [1931] for the plant proteases (papain, bromelin and yeast-proteinase). In later experiments it was established [Waldschmidt-Leitz, Scharikova and Schiiffner, 1933] that arginase, another enzyme concerned with intermediary protein metabolism, is also activated directly by metal complexes of SH compounds, for example, by reduced glutathione and ferrous or cuprous ions.The interpretation of these activations as a specific function of the sulphydryl group [Waldschmidt-Leitz, Weil and Purr, 1933] cannot be considered as valid, for we have found that other types of substances, as for example, ascorbic acid-Fe=, methylglyoxal-Fe=, and alloxan-Fe= exercise the same activating effect on arginase as is obtained with the sulphydryl-Fe= system (see Tables V, II and I). These newly demonstrated activators however possess in common the property of forming reversible redox systems. The hypothesis of a connection between arginase activation and the oxidation-reduction potential, for the establishment of which oxygen [Waldschmidt-Leitz, McDonald et al., 1933] or hydrogen [Edlbacher et al., 1933] is necessary, is in harmony with the experimental findings and accounts for the activations observed with the abovementioned compounds.Investigations reported by various writers during the past year have shown that in general these oxidations and reductions are regulated by reversible redox systems, which in the presence of available oxygen or hydrogen show variable potentials. Of significance to the investigation is the fact that the potential is determined not by the total amount of the system present but only by the ratio of the reduced to the oxidised phase. The total amount acts to a certain extent however as a buffer. With a constant ratio of reduced form to oxidised form, it is apparent that the redox potentials will vary also with the PH. With increasing alkalinity they become more negative (more intense), and with increasing acidity, less negative (less intense). The particular physiological significance of such biological oxidations and reductions is dependent on the one hand on the assumption of a capacity to absorb such reduction potential (specificity) and on the other hand on the intensity of such potential. It will