Elastic unloading [otherwise known as negative ventilator compliance (Cv) or proportional assist ventilation] is a new mode of assisted mechanical ventilation. The ventilator continuously measures the volume of spontaneous breathing (V) and adjusts the pressure at the airway opening in proportion to V. The quotient of pressure above the baseline end-expiratory level per unit of V (the gain of the assist) is constant at any point in time and can be preset. The apparatus used for this study can also generate clastic loading (positive Cv) by decreasing the pressure at the airway opening in proportion to V. This might be useful during the weaning process. This study compares measured values of total compliance of the combined lung-respirator system (Ctot) with values predicted according to theory, where 1/Ctot = 1/Cv + 1/Cv with C1 being the lung compliance. Respiratory mechanical data were derived from esophagcal pressure and airflow in eight anesthetized, intubatcd, spontaneously breathing cats. Different Cv, levels were set on the ventilator both before and after lung injury with xanthine oxidase. The difference (mean ± SD) between the measured and predicted Ctot was 1.4 ± 21.4% (healthy lungs) and −11.6 ± 14.1% (injured lungs) during unloading and 2.5 ± 7.5% (healthy lungs) during elastic loading. An elevation of Ctot decreased the expiratory airflow. Tidal volume increased slightly in healthy lungs and arterial Pco2 decreased. We conclude that the effects of Cv on the total compliance of the combined lung-respirator system can accurately be predicted.