ABSTRACT.The combined system of ventilator circuit, endotracheal tube, and lung commonly imposes a resistive load on spontaneous breathing efforts. It is possible to compensate for this positive resistance by a device generating a “negative ventilator resistance” (NVR),i.e.delivering a positive pressure during inspiration and a negative pressure during expiration in constant proportion to the instantaneous flow of the spontaneous breathing. The concept of NVR implies that there must not be any phase lag between flow and pressure signals. In eight anesthetized, intubated, spontaneously breathing rabbits (mean body wt 3570 g, range 2900‐4600 g), challenged either by aerosolized histamine or an extrapulmonary resistive load, lung mechanical data were calculated from esophageal pressure and flow signals. Each animal served as its own control with and without NVR. In a total of 39 experiments, NVR was applied in amounts between 1 and 15 kPa.s/L. During both types of additional resistive load, NVR immediately reduced the resistive work of breathing. There was a strong linear correlation between the amount of NVR applied and the decrease in total resistance, where the total resistance equals the resistive load on the animal's respiratory muscles (sum of the resistances of all components of the combined respirator‐lung system):r= 0.93,p< 0.001. The relationship between NVR and the drop in resistive work per mL of tidal volume was similar:r= 0.85,p< 0.001. Throughout the experiments, NVR operated in perfect synchronization with the animal's spontaneous breathing activity. (28: 79‐82, 1990)AbbreviationsCPAP, continuous positive airway pressureNVR, negative ventilator resistancePes, esophageal pressurePm, pressure at the mouthpiece levelRext, extrapulmonary resistive load&OV0622;, flow