A system of catalytic reactions is proposed as the basis of respiration. According to this system the reserve carbohydrates, A, are hydrolyzed to normal hexoses, B, which by activation form heterohexoses, C. These by glycolysis form intermediate products, D, which are the reactants in respiration. In an atmosphere of N these products respire to form the CO2 of N respiration and alchol. In the presence of O2 they respire to form the CO2 of O respiration, H2O, and oxidative anabolic products, OA. This system provides a plausible interpretation of the variations of CO2 pro-duction reported previously. A measure of the rate of glycolysis is obtained from the variations of CO2 production in N and in various concentrations of O2. It is concluded that glycolysis proceeds at a greater rate in air than in N and is accelerated by further rise of O concentration.