It is well known that nitrite is formed from nitrate by the catalytic action of the purified enzyme of liver with acetaldehyde, but not reduced further. However, less amounts of nitrite were accumulated in the reaction mixture when FAD had been supplemented. At first, it w as observed that the colorimetric estimation of nitrite was not so significantly inhibited by FAD and that nitrite could not be destroyed even if it had been boiled for 5min. with FAD. On adding FAD in the enzyme system, the decrease of nitrite was brought about but not stimulated by acetaldehyde or DPN, which are the effective hydrogen donor for the nitrate reduction by the same enzyme. The rate of decrease of nitrite was correlated with the amount of enzyme used and with the reaction time. Furthermore, the enzyme was most reactive at 50℃ and at pH 6.0. These optimum points of temperature and pH coincide roughly with those of the nitrate reductase. Thus, it was suggested that the enzymatic diminution of nitrite might be attributed to the action of the nitrite reductase which was made reactive by addition of FAD.