Glycolate and glyoxylate stimulated 100% to 300% the rate of oxygen evolution by Scenedesmus in the light in the absence of added carbon dioxide. This stimulation occurred either aerobically or anaerobically, and was sensitive to CMU. Aerobic dark respiration was stimulated 25% to 100% by glycolate. This phenomenon was best demonstrated with synchronized Scenedesmus at the stage of cell division. For glycolate stimulation of oxygen evolution, a dark preincubation of 1 minute or less was necessary. In comparative test with other compounds of metabolism and photosynthesis, the stimulation of oxygen evolution was greatest by glycolate and glyoxylate. In a proposed scheme glyoxylate serves as a terminal hydrogen acceptor from NADPH produced by photosynthesis, and it thereby stimulates oxygen evolution when carbon dioxide is not available. Transformation of glycolate to glyoxylate in these cells would have to occur in the absence of oxygen.