Using both the conventional Warburg and the spectrophotometric method, the sites of action of ethylcarbamate (EC) and some other narcotics on malic dehydrogenase and the linked electron transport system in rat heart and brain homogenates and liver mitochondria, were studied. The results obtained were as follows:1. The addition of EC (over 0.1M) caused inhibition of oxygen uptake in malate oxidation by a heart muscle homogenate in the presence of cytochrome c (CYT. C) or methylene blue. On addition of nicotinamide adenine dinucleotide (NAD) the inhibition in the presence of the latter decreased significantly.2. Addition of EC caused inhibition of the reduction of CYT. C, 2, 6-dichlorophenol- indophenol and ferricyanide in malate oxidation in the presence of cyanide. On further addition of NAD the degree of these inhibitions decreased.3. In the presence of cyanide, the reduction of CYT. C by NADH was inhibited by 51% with EC (0.3M), while in the absence of cyanide, the reduction was apparently enhanced owing to the inhibitory action of EC on the reoxidation of reduced CYT. C by oxygen.4. In the presence of EC, the reduction rate of NAD in malate dehydrogenation increased, and oxidation of NADH by oxaloacetate was not inhibited.5. Of the systems tested the inhibition of NADH oxidase activity (the oxidation of added NADH by oxygen) by EC was the largest (FIG. 4).6. The oxidation of para-phenylenediamine (PPD) with added CYT. C in the heart, liver and brain preparations was inhibited by EC (0.3M) and chloretone (CL, 0.01M) by 40 to 50%, though in the liver and brain the inhibition was 10% or less than in the heart preparation, while amytal (0.005M) and veronal (0.04M) caused little or no inhibition. On the other hand, in the absence of added CYT. C all inhibitions decreased by about 40%.7. The oxidation of ascorbic acid (VC) with added CYT. C was inhibited similarly by the narcotics, but the inhibitions were significantly lower than those of PPD oxidation.8. The oxidations of added reduced CYT. C in the heart and liver preparations, like those of VC, were inhibited by EC.From these results, the mechanism of action of EC on malic dehydrogenase and the linked electron transport system was discussed in relation to CYT. C, leading to the following conclusions:Narcotics, ethylcarbamate and chloretone, inhibit not only the cytochrome c reduction system, but also the cytochrome c oxidation system (cytochrome oxidase) of the electron transport system linked to malic dehydrogenase.