The close balance between the O2 requirement to perform exercise and the O2 supply was analyzed. A non-uniform capillary PO2 can result in anaerobic metabolism in some muscle fibers despite an apparently adequate mean capillary PO2. The pattern of lactate increase for constant work rates and incremental exercise is described. Lactate increases without an increase in pyruvate at a threshold work rate above which the lactate/pyruvate ratio increases. The latter decreases immediately at the start to recovery. From simultaneous measurements of arterial lactate and pyruvate during exercise and recovery, we conclude that the lactate increase at the lactate threshold is consequent to a change in redox state rather than a mass action effect.