Inspiratory mechanical loads were applied to the airway continuously for 5 min in healthy young adult volunteers maintained in a near steady-state of halothane anesthesia 1.1 MAC. The loads, both flow resistive and elastic in nature, had been selected to reduce the first loaded tidal volume approximately 10, 30, or 50%—these being designated “small,” “medium,” and “large” loads, respectively. The actual magnitudes of resistive load were 8 ± 1, 21 ± 3, and 48 ± 6 cmH2O.1−1s, and of elastic load 6 ± 1, 18 ± 1, and 41 ± 5 cmH2O. 1−1(means ± SEM). All loads caused an immediate reduction of ventilation proportional to the size of the load. This was followed by a gradual recovery of ventilation toward control values over approximately 2 min and then nearly stable ventilation for the rest of the loading period. Respiratory frequency was unchanged throughout. At 5 min of loading, ventilation and Pco2had been nearly steady for 3 min and O2uptake and CO2output at the airway were unchanged from control, suggesting the establishment of a near steady respiratory state. With the small and medium loads of both types, ventilation and Paco2in this near steady-state were not detectably different from control. With the large loads, however, ventilation was significantly reduced and Paco2slightly increased. The end-expiratory position of the chest wall and the relative contributions of the rib cage and abdomen-diaphragm to ventilation, as estimated by antereroposterior chest wall magnetometers, were not consistently altered by any load. The small elastic loads reduced the “effective” intrinsic elastance at the end of the loading period. Loading did not increase the ratio of dead space to tidal volume (VD/VT) or the alveolar-arterial oxygen tension difference (P(A-a)o2). It is concluded that in healthy young humans anesthetized with halothane, steady-state compensation for inspiratory flow-resistive loads up to about 21 cmH2O.1−1s and inspiratory elastic loads up to 18 cmH2O · 1−1is virtually complete. Ventilatory compensation for these loads is due to nonchemical mechanisms, which in the case of small elastic loads reduce intrinsic elastance and thereby improve the performance of the ventilatory pump.