In 1949, Hogeboom (1) reported that in rat liver homogenates which were prepared in 0.88 M sucrose and separated into four fractions by differential centrifugation, 32 and 58 per cent of the total D P N H 1 cytochrome c reductase activity of the original homogenates were recovered in the mitochondria and microsomes, respectively. He also found that, based on activity per mg. of nitrogen, the enzyme was concentrated 3.3 times in the microsome fraction and only 1.3 times in the mitochondria with respect to the homogenate. In 1951, as part of a study of the D P N H cytochrome c reductase of rabbit liver and heart mitochondria, Eichel (2) observed a 1.6-and 2.9-fold concentration of the enzyme in mitochondrial fractions prepared from isotonic sucrose homogenates of liver and heart, respectively. Cytochrome c oxidase assays on these (and other) fractions and the original homogenates confirmed the mitochondrial nature of the isolated fractions. Subsequently, Brody et al. (3) studied the distribution of D P N H cytochrome c reductase in various fractions obtained from homogenates of rat brain cerebral cortex. In contrast to the results with rat liver, they found a 2.8-fold concentration of the enzyme in the mitochondria with respect to the original homogenate, while the specific activity of the microsomal fraction was slightly less than that of the original homogenate. No data were given for the recovery of total enzyme activity in each of the particulate fractions. Strittmatter and Ball (4) and de Duve el al. (5) have also studied the distribution of the reductase in rat liver fractions. Their results are in essential agreement with those reported previously.