DURING the course of an investigation of the oxidation of nitrite by the nitrite-oxidizing bacterium, Nitrobacter, it was noted that the addition of sodium nitrite but not sodium nitrate to heavy cell suspensions caused the immediate appearance of reduced cytochrome bands at 590, 551 and 513 mµ when viewed in the hand spectroscope1. This phenomenon, which has been noted previously by Lees and Simpson2, is due to the passage of electrons through the respiratory chain when nitrite is oxidized to nitrate. Whether or not nitrifying bacteria respire in response to substrates other than their energy source has been disputed, although Bomeke3 reported a slight stimulation of the respiration of Nitrobacter when certain metabolites were added to cell suspensions in the Warburg respirometer. If some reduced substance is oxidized by resting cells and if a cytochrome-linked pathway is employed for the oxidation of all substances which can permeate the cell, then any substance which might increase the rate of respiration in the Warburg respirometer should also cause a reduction of the cellular cytochromes. The validity of this rationale was tested in the following manner.