Patients with left-to-right shunt congenital heart disease may develop pulmonary hypertension. Perioperative mortality of these patients is high due to abnormal vasoreactivity of the pulmonary artery (PA). We studied the changes in the PA induced by high pulmonary blood flow in rats with aortocaval fistula. Eight weeks after surgery, morphological changes of the PA were studied and vasomotor function was assessed by isometric force recording. Expression of endothelial nitric oxide (NO) synthase (eNOS), VEGF, and cyclooxygenase-2 (COX-2) proteins and levels of cGMP in the PA were analyzed. Rats with high pulmonary blood flow developed pulmonary hypertension, medial thickening, and increasing of internal elastic lamina and basement membrane in the PA. When compared with sham-operated animals, rats with fistula had significantly increased contractions in the PA, whereas relaxations to acetylcholine and NO donor were reduced. Concentrations of cGMP were reduced in the PA of rats with pulmonary hypertension (18.4 Ϯ 3.3 vs. 9.4 Ϯ 1.7 pmol/mg protein; P ϭ 0.04). The altered vasomotor function was normalized by treatment with indomethacin. The PA of rats with fistula expressed higher levels of eNOS, phosphorylated eNOS, and COX-2. Sustained high PA blood flow in rats causes pulmonary hypertension that is morphologically and functionally identical with patients with flow-induced pulmonary hypertension. Abnormal vasomotor function of the PA in these animals appears to be mediated by reduced availability and the biological effect of endogenous NO and the high production of vasoconstrictor prostanoids. Increased eNOS and phosphorylated eNOS are most likely the adaptive changes in response to an increase in PA pressure secondary to high blood flow. guanosine 3Ј,5Ј-cyclic monophosphate; cyclooxygenase-2; endothelium; endothelial nitric oxide synthase PULMONARY HYPERTENSION, secondary to increased pulmonary blood flow, may develop in patients with left-to-right shunt congenital heart defects or systemic arteriovenous shunt, such as arteriovenous access for hemodialysis (21,26,37,53). The characteristic vascular changes include intimal hyperplasia/ fibrosis, medial hypertrophy, extensive extracellular matrix modulation, and, in more severe cases, the formation of plexiform lesion (21). These changes lead to decreased compliance of pulmonary vasculature and changes in vasoreactivity of pulmonary arteries (PAs) (34). Patients with flow-induced pulmonary hypertension have significantly higher perioperative mortality and a lower long-term survival after a cardiac operation (3,24,44). During and after an operation, these patients often exhibit fatal pulmonary vasoconstriction (2, 45). Patients with flow-induced pulmonary hypertension also showed significantly impaired endothelium-dependent relaxation in the PAs, whereas endothelium-independent relaxation is relatively preserved (10,17,29).The underlying pathogenesis of flow-induced pulmonary hypertension remains poorly understood (4). The patients with left-to-right congenital heart...