The activity of nitrate reductase in the fowl liver homogenate was estimated. It was completely inactivated by heating at 90℃ for 2 minutes and at 80℃ for 5 minutes. However, approximately 15 per cent of the original activity was retained at 70℃ and the slight diminution was observed when the enzyme had been heated at 60℃. The enzyme had a tendency to decrease linearly its activity at the initial stage of heating and then to reach gradually to certain final values according to the temperatures. The optimum pH of the nitrate reductase was about 5.8 coinciding with those of mouse and cattle liver enzymes, though the pH range in which the nitrate reductase is acting was much broader than that of both mammalian reductases. Especially, wide plateau of optimum pH existed between 6 and 7 when the enzyme assays were carried out using phosphate buffer. The enzymatic activity was fallen off considerably by dialysis and restored with the dialysate and boiled extract of liver. Moreover, the activity was also recovered some extent by addition of malate, glutamate, succinate, tartarate, fumarate, lactate, maleiate, malonate, and citrate, since native substances which are acting most predominatingly as hydrogen donator in the liver had been lost by dialysis. However, reactivation with sugars was not observed. The rate of nitrate reduction of the dialysed enzyme was accelerated exceedingly by acetaldehyde which has almost no or slight inhibitory effect on the reductase of the original homogenate. In connection with aldehyde dehydrogenase which was demonstrated so far to be able to reduce nitrate, therefore, some discussions were performed on the active principal reducing nitrate. It was observed that reactivation of dialysed nitrate reductase was attained by the conjunction of raw flavin extract of liver which contains three components FAD, FMN, and ribpflavin. Moreover, it was also suggested that FAD may concern with reduction but both the laters not. From the fact that thiourea, oxine, EDTA, NaN3, KCN, PCMB, monoiod acetic acid, Cu++, Ag+, and hydroxylamine inhibited the reduction of nitrate, the participation of metal and SIi group in the reaction was presumed.