The effect of intermittent high altitude (IHA) hypoxia on the myocardium and lesser circulation was investigated in adult male Wistar rats. IHA can induce intermittent pulmonary hypertension and right ventricular hypertrophy in a relatively short time. Even marked pulmonary hypertension, right ventricular hypertrophy, and pulmonary vascular changes can be normalized when rats are removed from the hypoxic atmosphere. At the beginning of the exposure to IHA acute myocardial necrotic changes were found; prolongation of IHA did not lead to further acute lesions. Experimentally induced CO polycythemia leads to mild pulmonary hypertension; IHA-induced pulmonary hypertension may, thus, be partly due to polycythemia. Beta blocking agents are able to decrease chronic hypoxic pulmonary hypertension, hypertensive changes in the pulmonary circulation, the degree of right ventricular hypertrophy, and necrotic myocardial changes.