The addition of rat-liver mitochondria to rat-brain supernatant fraction depressed the accumulation of lactic acid under aerobic conditions. Glucose utilization was also lowered, but not in proportion to the depression in lactate accumulation seen in the presence of the mitochondria. Rat-liver mitochondria stimulated glycosis when added to a rat-liver supernatant fraction if glucose was the sole substrate. In the presence of hexose monophosphates mitochondria inhibited the accumulation of lactate. Mitochondria from pigeon-breast muscle or heart depressed glycolysis by a pigeon-breast muscle supernatant fraction under both aerobic and anaerobic conditions. None of the effects of the mitochondria appeared to be due to either inhibition of phosphofructokinase or the Pasteur effect. The effects of liver mitochondria are attributed to electron transport uncoupled from oxidative phosphorylation and those of the muscle mitochondria to phosphatase activity.