In homogenates of the Walker 256 rat tumor and of rat brain and rat liver there are two distinct mechanisms for phosphorylation: (1) particulate elements dependent on oxidation of succinate or other substrates of the tricarboxylic acid cycle, which are suppressed by 10-5 [image] dinitrocresol, and (2) water-soluble or at least water-borne systems glycolyzing hexose diphosphate, which pass into the supernate on centrifuging and which are completely unaffected by concns. of dinitrocresol from 100- to 500-fold those required to suppress aerobic phosphorylation. The oxidative particulate system is proportionately highest in liver homogenates, intermediate in brain homogenates, and lowest in tumor homogenates. The anaerobic, glycolytic, water-soluble phosphorylating system is preponderant in tumor homogenates , intermediate in brain homogenates, and lowest in liver homogenates. Comparative expts. conducted under aerobic and anaerobic conditions confirm the conclusion that dinitrocresol suppresses aerobic phosphorylation, but does not affect anaerobic phosphorylation, even when used at high concns.