Clinical observation by the authors suggests that small changes in PaCO2cause significant alterations in pulmonary hemodynamics after cardiac surgery. To examine this, the authors induced moderate ventilatory hypocarbia (Paco2= 30.7 ± 0.4 mmHg, mean ± SD) in eight patients early after coronary artery bypass surgery. Normo-carbia (40.6 ± 0.5 mmHg) and hypercarbia (51.5 ± 0.5 mmHg) were then induced by adding CO1to the inspired gas. Standard hemo-dynamic measurements were performed at each stage. In four of the patients, CO1exposure was subsequently withdrawn in reversed order. There were no clinically significant changes in systemic hemodynamics. Despite unchanged pulmonary blood flow, in all patients pulmonary artery pressure increased with increasing PaCO2An almost two-fold and three-fold increase was observed in the mean pulmonary vascular resistance (PVR) and pulmonary diastolic gradient (pulmonary artery diastolic pressure-pulmonary capillary wedge pressure), respectively, when PaCO2was changed from hypocarbia to hypercarbia. The changes in the pulmonary diastolic gradient correlated (r = 0.77, p < 0.001) with the changes in the PVR. The pulmonary vasoconstrictor response was reversible with CO2washout. Avoidance of even moderate hypercarbia, therefore, seems advisable in the early postperfusion stage because of a further potential impedance to right ventricular ejection. Frequent measurement of pulmonary diastolic pressure gradient provides a useful method of rapid estimation of the resistance to flow in the pulmonary vascular bed.