An increase in left atrial pressure (Pla) has been reported to either inhibit or not affect hypoxic pulmonary vasoconstriction in intact dogs. We investigated mean pulmonary arterial pressure (Ppa)-flow (Q) relationships at low and high fixed Pla and Ppa-Pla relationships at fixed Q in piglets, which are known to present with a stronger hypoxic pulmonary pressor response than dogs. Seven piglets were anesthetized; equipped with balloon catheters in inferior vena cava and left atrium to control Q and Pla, respectively; and ventilated alternatively in hyperoxia [fractional concn of O2 in inspired air (FIO2) 0.4] and hypoxia (FIO2 0.12). In all experimental conditions, Ppa-Q plots were best described by a linear approximation with extrapolated pressure intercepts (Pi) not different from Pla. Hypoxia increased slope but not Pi of Ppa-Q plots. An increase in Pla from 8 to 17 mmHg induced a parallel shift of Ppa-Q plots to higher Ppa in hyperoxia but did not affect Ppa-Q plots in hypoxia. In hyperoxia, an increase in Pla at constant Q induced an approximately equal increase in Ppa, whereas in hypoxia there was no effect. The hypoxia-induced increase in Ppa was blunted by increased Pla at all levels of Q studied. We conclude that in anesthetized piglets at fixed Pla hypoxia increases the slope of Ppa-Q plots without affecting Pi and an increase in Pla inhibits hypoxic pulmonary vasoconstriction. The results suggest that no closing pressure higher than normal Pla contributes to hyperoxic or hypoxic Ppa in the intact porcine pulmonary circulation.