The effect of chronic hypoxia (barochamber, 8 h per day, 5 days a week, stepwise up to 7000 m) on some cardiac functional and metabolic parameters was compared in rats acclimatized either from the 4th day or the 12th week of postnatal life. Pulmonary hypertension and right ventricular enlargement were found in both age group. Whereas in young hypoxic animals the weight of the right ventricle increased linearly with the right ventricular pressure, in adult high altitude exposed rats this relation could not be proved. High altitude induced significant increase of concentration of collagenous proteins: in adult animals increased collagen III only, in young also collagen I. The changes of the energy metabolism were similar in both age groups: activity of enzymes of the lactate metabolism and glucose phosphorylation increased; on the other hand, catabolism of fatty acids decreased. Chronic hypoxemia significantly influenced also the myocardial metabolism in children with congenital heart disease; the aerobic capacity was decreased both in the atrial and ventricular tissue; atrial changes were even more pronounced. The type of changes was, however, different from the rat heart.