The activity of the nitrate reductase could not be detected in the nuclei which had been isolated from the fowl liver by the citric acid method. Then the liver was fractionated with the isotonic sucrose solution by the differential centrifuge method and the intracellular distribution of the nitrate reductase was surveyed by estimating the activity of the mitochondrial, microsomal and supernatant fractions. It was found that most of the activity was contained in the supernatant, while it was scarcely detected in the former two particles. Since native hydrogen donators were dissolved in cell sap, it was suggested that the nitrate reduction can be performed within the cytoplasm of the liver cell. However, the mitochondria could mediate transfer of hydrogen between some donators and the nitrate reductase. The absorption spectrum of the supernatant fraction was estimated and found to be very similar to that of the purified preparation. The former, however, have two more feeble peaks at 530 and 562 mμ, which were extinguished by dialysis. These observations agreed well with those concerning the livers of mouse and cattle.
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