The effect of intermittent altitude hy- poxia simulated in a hypobaric chamber (7,000 m, 8 h daily, 5 days a week) on the lesser circulation and heart weight was studied in rats. A marked chronic pulmonary hypertension and hypertrophy of the right ventricle after 24 exposures was found. No further rsignificant increase after 60 exposures was reached. Signs of right heart failure were found both after 24 and 60 exposures. 70 days after the removal of the rats from the hypobaric chamber and recovery in normoxic conditions, the right ventricular systolic pressure and right ventricular weight did not differ from controls showing reversibility of pulmonary hypertension and right ventricular hypertrophy even in animals with signs of right heart failure.