(1) Reduction of nitrite, nitric oxide and nitrous oxide by Alcaligenes faecalis grown anaerobically in a nitrate-containing medium was examined. (2) When lactate was used as electron donor, the resting cells produced nitrogen from nitrite. Azide (10−2 M) decreased the rate of gas formation and the gas formed was identified as nitrous oxide. Cyanide (10−4 M) also inhibited gas formation, and the gas formed was identified as nitric oxide and nitrous oxide. Conversion of nitrous oxide to nitrogen was inhibited completely in the presence of KCN or NaN3. The reduction of nitric oxide to nitrous oxide was strongly inhibited by KCN. Hydroxylamine was not utilized as a source of nitrogenous gas product, but was a strong inhibitor for nitrite reduction. (3) Cell-free extracts could produce only nitrous oxide from nitrite or nitric oxide. Phenazine methosulfate with ascorbate was found to be a good electron donor for nitrite and nitric oxide reduction. (4) Cytochrome cd was suggested to function as nitrite reductase which converted nitrite to nitric oxide. (5) Nitric oxide reductase, which catalyzed the reduction of nitric oxide to nitrous oxide and existed mostly in the particulate fraction, was partially obtained in the soluble fraction. It was suggested that an inhibitory substance capable of masking nitric oxide reductase activity was present in the soluble fraction.