Dehydrogenase-catalyzed reactions are widely used in the quantitative analysis of chemical substances. In these reactions the common product is NADH. By including diaphorase and a redox couple (ferricyanide and ferrocyanide ions) in the assay medium, the production of NADH becomes detectable with a redox electrode. The relationship between the redox potential and the substrate concentration can be simplified by reducing the number of variables. With the method described here the Nernst equation reduces to E = K - S log[con.], where E is the redox potential, K and S are the intercept and the slope of a straight line respectively, and [con.] is the sample concentration. The method was successfully applied to the analysis of blood alcohol, and shows promise for assaying many other tissue constituents.