We investigated a causal role for coronary endothelial dysfunction in the development of monocrotaline (MCT)-induced pulmonary hypertension and right heart hypertrophy in rats. Significant increases in the pulmonary pressure and the right ventricular (RV) weights did not occur until 3 weeks after 60 mg/kg MCT injection (34±4 mmHg vs. 19±2 mmHg and 37±2% septum plus LV weight vs. 25±1% in controls, respectively). Isolated right coronaries showed significant decreases in acetylcholine-induced NO dilation in both 1-week (33±3% with 0.3 µM, n=5 rats) and 3-week (18±3%; n=11) MCT rats as compared to the controls (71±8%, n=10). Septal coronary arteries (SCA) showed a smaller decrease in acetylcholine dilation (55±8% and 33±7%, respectively vs. the controls of 73±8%). No significant change was found in the left coronaries (LCA, 88±6% and 81±6%, respectively vs. 87±3% in controls). The L-NAME induced vasoconstriction, an estimate of the spontaneous endothelial NO mediated dilation, was not significantly altered in MCT-treated SCA nor LCA, but was increased in the RCA after 1 week MCT (-41±6%) and decreased after 3 weeks (-18±3% vs. -27±3% in controls). A marked enhancement to 30 nM U46619-induced constriction was also noted in the RCA of the 3-week MCT (-28±6% vs. -9±2% in controls), but not the 1-week (-12±7%). Sodium nitroprusside-induced vasodilation was not different between the control and MCT rats. Taken together, our findings show that a selective impairment of right, but not left, coronary endothelial function is associated with and precedes the development of monocrotaline-induced pulmonary hypertension and right heart hypertrophy in rats.