There is now increasing evidence from the experimental and clinical setting that therapeutic hypercapnia from intentionally inspired carbon dioxide (CO(2)) or lower tidal volume might be a beneficial adjunct to the strategies of mechanical ventilation in critical illness. Although previous reports indicate that CO(2) exerts a beneficial effect in the lungs, the pulmonary vascular response to hypercapnia under various conditions remains to be clarified. The purpose of the present study is to characterize the pulmonary vascular response to CO(2) under the different conditions of pulmonary hypertension secondary to increased pulmonary blood flow and secondary to hypoxic pulmonary vasoconstriction. Isolated rat lung (n = 32) was used to study (1) the vasoactive action of 5% CO(2) in either N(2) (hypoxic-hypercapnia) or air (normoxic-hypercapnia) at different pulmonary arterial pressure levels induced by graded speed of perfusion flow and (2) the role of nitric oxide (NO) in mediating the pulmonary vascular response to hypercapnia, hypoxia, and flow-associated pulmonary hypertension. The results indicated that inhaled CO(2) reversed pulmonary hypertension induced by hypoxia but not by flow alteration. Endogenous NO attenuates hypoxic pulmonary vasoconstriction but does not augment the CO(2)-induced vasodilatation. Acute change in blood flow does not alter the endogenous NO production.