The reducing power of cells of Bact. lactis aerogenes has been measured by dye reduction tests with resting and with growing cultures in media containing as nitrogen sources ammonium salts or nitrate, and under aerobic and anaerobic conditions. Changes in growth rate have been correlated with changes in the reducing power accompanying transitions from one medium to the other or from anaerobic to aerobic conditions and vice versa. Two types of adjustment play a part in the observed phenomena: (a) rapid changes in concentrations of active intermediates, (b) slow adaptive modifications of enzyme systems in response to needs of the cell. The former are exemplified inter alia by the rapid fall in reducing power and growth rate when an anaerobic nitrate culture is aerated. The latter include the development of extra reducing power when the cell has to use nitrate as a source of nitrogen and of still more when the nitrate has to be reduced in presence of oxygen.