On 12 subjects of different muscular fitness the rate of lactic acid appearance in blood, while performing the same supramaximal exercise has been determined, together with the maximal performance time and the maximal L.A. concentration in blood. The rate of increase of lactic acid is higher in the less fit than in the athletic subjects, to compensate for the lower oxygen consumption. In all subjects the appearance of L.A. in the blood is delayed: at the onset of the exercise other anaerobic processes (alactic) supply the energy required, and only when these are exhausted L.A. formation enters into play. The energy due to lactic acid corresponds to 37±3.5 ml of O2 per g of lactic acid increase in 1 l of blood, or 50 ml of O2 (or 250 cal) per g of L.A. produced from glycogen. The maximal amount of the lactacid debt is equivalent to about the maximum oxygen consumption in 1 min. A simple relation is found between the time of performance in supramaximal exercise and the maximum oxygen consumption.