Resistive and/or elastic unloading (a negative ventilator impedance, otherwise termed proportional assist ventilation) may be a useful means to assist spontaneous breathing. This only applies if the ventilator accurately provides pressure changes at the airway opening (Pao) proportional to the instantaneous flow and/or volume signal of spontaneous breathing and no significant phase lag. We designed such an infant ventilator, which controls the Pao, by a negative feedback loop, and superimposes a second positive feedback circuit to generate unloading. To test this mode and the ventilator's accuracy in performing the synchronized pressure changes, we examined the functional residual capacity (FRC). We hypothesized that unloading by itself would not alter FRC because Pao, should return to a preset baseline at end‐expiration, and furthermore, that FRC could be actively altered by changing the baseline. Five anesthetized, tracheotomized, spontaneously breathing rabbits [respiratory system compliance 22.4 ± 4.6 mL/kPa (mean ± SEM)] were exposed to end‐expiratory Pao, levels of 0, 0.2, 0.4, and 0.6 kPa. At each of these levels a period of regular continuous positive airway pressure (CPAP) was alternated with a period of unloading (−40 mL/kPa ventilator compliance combined with −3 kPa/Us ventilator resistance). FRC measured by a sulfur hexafluoride washout technique was virtually identical on CPAP and during unloading at equal end‐expiratory Pao, (difference, 1.41% ± 0.95%), but FRC increased upon elevation of the end‐expiratory Pao, by 29.4 2 3.6 mL/kPa on CPAP and 30.2 ± 3.2 at unloading (difference NS). We conclude that FRC is not destabilized by unloading, but that during unloading, as during CPAP, it depends on the end‐expiratory Pao. Pediatr Pulmonol. 1993; 16:170–176. © 1993 Wiley‐Liss, Inc.