We characterize the determinants of pulmonary arterial flow ( Qpa ) variation during spontaneous breathing ( SPONT ) and matched tidal volume intermittent positive-pressure breathing (IPPB) in 14 pentobarbital-anesthetized closed-chest canine preparations in which Qpa is measured by an electromagnetic flow probe around the pulmonary artery. Pressures are recorded from the juxtacardiac pleural space (Ppl), right atrium (Pra), and pulmonary artery. Spontaneous inspiratory efforts increase transmural Pra (Pra - Ppl) and right ventricular stroke volume ( SVRV ) but decreases Pra, whereas IPPB inspiration has the opposite effect. However, the relationship between instantaneous changes in SVRV and transmural Pra during SPONT and IPPB define a common "instantaneous" right ventricular (RV) function curve independent of mode of ventilation or phase or respiratory cycle, and this curve is similar to one generated by volume infusion measured at end expiration. Pulmonary vascular resistance changes during ventilation are small (less than 15%) and do not affect RV performance as noted by similar instantaneous RV function curves for SPONT and IPPB with Mueller and Valsalva maneuvers, respectively. Thus variations in Qpa during ventilation represent matched changes in RV filling pressure induced by phasic changes in venous return.