Nitrogen was formed from dimethyl-p-phenylenediamine (DMPD) and nitrite (DMPD-nitrite system) by Pseudomonas denitrificans. The formation of N2 in the DMPD-nitrite system was not inhibited by 10−2M NaN3 which perfectly suppressed the formation of N2 in the normal denitrification of the lactate-nitrite system. In the crude cell extract, the activity of N2 formation in the lactate-nitrite system was lost entirely, while N2 formation in the DMPD-nitrite system remained intact. DMPD inhibited the reduction of N2O to N2. NO reacted with DMPD nonenzymatically, producing N2. From hydroxylamine and nitrite the intact cells produced N2O, and in the concomitant presence of lactate as hydrogen donor, produced N2O and NO instead. When a lower concentration of hydroxylamine was used, it was utilized only a little as a nitrogen source but it acted as an inhibitor, where N2O, N2 together with a small amount of NO were produced from nitrite and lactate. The formation of N2 from N2O was strongly suppressed in the presence of both NO and NH2OH. The reduction of NO to N2O, the second enzymatic step of nitrite reduction, was also sensitive to hydroxylamine. A possible scheme of the process of gas formation from nitrite under various conditions is presented.