Malate oxidation by mitochondria prepared from stage γArum spadices is inhibited by ADP. When oxaloacetate is also present, the effect of ADP is to increase the rate of oxygen uptake to one well in excess of the rate with malate alone. Arum mitochondria cannot oxidize oxaloacetate alone, but when increments of ADP or other nucleotides are added with oxaloacetate, oxygen uptake begins and continues for an extended period at a constant rate. The rate is proportional to the amount of nucleotide added. The stimulation of oxaloacetate oxidation by nucleotides increases in the order AMP < ADP < ATP, and Mg2+, but not PO2−3, is required for the stimulatory effect of the nucleotide. These responses to nucleotides are thought to be occasioned by the role of these substances as effectors of an enzyme or enzymes concerned with the metabolism of malate and oxaloacetate rather than involving the phosphorylative capacity of the mitochondria because the results are independent of the presence or absence of uncoupling substances.