The pressure volume relationship of the respiratory apparatus during maximum inspiration and expiration, made as quickly as possible through different resistances, has been determined. The time course of the alveolar pressure during such events has been described. The maximum work done by the respiratory muscles increases as the resistance increases and as the speed of muscle shortening decreases. The relationship between maximum work and mean velocity or, at a given volume, between alveolar pressure and instantaneous flow is nearly linear. The discrepancy between the relationship found and the hyperbolic one described by the characteristic equation of the muscles is ascribed to the geometrical and anatomical complexities of the respiratory apparatus. The results, however, show that the air flow is limited by the rate with which the muscles are able to mobilize chemical potential energy for the performance of work. Submitted on December 10, 1959