The present study characterized metabolic changes in the heart associated with long-term exposure to hypoxia, a potent stimulus for pulmonary hypertension and right ventricular hypertrophy. When anesthetized rats adapted to chronic hypoxia spontaneously respired room air, their mean right intraventricular peak systolic pressure (RVSP) was twice that in normal control animals with the same arterial PO2. RVSP was linearly related to right ventricular mass (r = 0.78). Oxidative capacity (O2 consumption) of homogenates of right and left ventricles from both groups of rats was measured with one of the following substrates: pyruvate, glutamate, acetate, and palmitoyl-L-carnitine. Oxidation of all substrates was significantly greater in the left than in the right ventricle in normal rats but not in hypoxia-adapted animals, where it was the same, within the experimental error. O2 consumption by the left ventricle was greater in control than in experimental rats, but right ventricular O2 consumption was similar in the two groups. Maximal reaction velocity of cytochrome-c oxidase was about the same in the two ventricles, and there were no significant differences between control and hypoxia-adapted animals. HPLC analyses showed significantly higher aspartate levels and aspartate-to glutamate concentration ratios in both ventricles of hypoxic rats than in corresponding tissues from controls, indicative of a decreased flux through the malate-aspartate shuttle under conditions of O2 limitation. Myocardial glutamine levels were lower in hypoxic rats, and glutamine-to-glutamate concentration ratios decreased, although primarily in the pressure-overloaded right ventricle. These findings indicate that normal energy metabolism in the left ventricle differs from that in the right and that the differences, particularly those of amino acid metabolism, are markedly influenced by chronic exposure to hypoxia.