The weight of the ventricles and septum in formalin-fixed hearts were determined in six animal species living continuously at altitudes between 10,000 and 15,400 ft in the central Peruvian Andes. Control studies were made of a similar number of hearts obtained from sea-level animals. Guinea pigs, rabbits, dogs, lambs, pigs, and steers all exhibited a moderate hypertrophy of the right ventricle roughly equivalent to a 25% increase in weight. A lesser degree of septal hypertrophy occurred in all animals except steers. The data suggest that a moderate degree of pulmonary hypertension is the probable cause of the right ventricular hypertrophy. In steers this would be roughly the equivalent of a mean pressure of 35 mm Hg at 11,800 ft compared with sea-level pressures of 24 mm Hg.