The study about the effect of some purines on the nitrate reductase of the fowl liver was carried out comparatively according to that with the mouse liver enzyme. The nitrate reductase was exceedingly inhibited by adenine, guanine, hypoxanthine and xanthine, the precoursors of uric acid in the tissue. On the contrary, the activity of the same enzyme to decolor methylene blue was enhanced by these purines, since the liver enzyme preparation contains the xanthine oxidase. Therefore, it was presumed that the active centre of the nitrate reductase may be different from that of the xanthine oxidase which had been so far demonstrated to able to reduce nitrate. 8-Azaguanine, in which 8'C of guanine was replaced by N, retarded both reductions of dye as well as nitrate. However, N-methylated purines such as theophylline and caffeine less or no effective in inhibiting the enzymatic activity. Thus it might be supposed that Ns of 1, 3 and 7 of the purine ring take an essential part in the inhibition of the nitrate reductase. There were only a few discrepancies between both results with the fowl and the mouse liver enzymes, although the results coincided well each other. By adenine and guanine, the decoloration of methylene blue with the fowl enzyme was accerelated while that with the mouse was inhibited. In addition, regarding the promotion of the dye bleaching by hypoxanthine and xanthine, the degree of the activation with the fowl liver enzyme was increased in proportion to the amounts of purines added in the reaction mixture, contrary to the behavior with the mouse enzyme which suggest the substrate inhibition. In principle, however, it was demonstrated that the characteristic difference of the nitrate reductase of the fowl liver from that of the mouse liver could be scarcely established.