The pulmonary trunk, muscular arteries, and arterioles of male Sprague-Dawley rats exposed to simulated high altitude hypoxia (380 mm Hg) for six weeks were studied for glycogen localization. As early as three days exposure time, glycogen particles were mobilized in the smooth muscle cells of muscular arteries and arterioles. Weekly sacrifice of animals showed increasing preferential accumulation of glycogen, near the sarcoplasmic reticulum, in the subsarcolemmal region, adjacent to micropinocytotic vesicles, and near the mitochondria in the smooth muscle cells of muscular arteries and arterioles. However, in the smooth muscle cells of the pulmonary trunk and newly muscularized vessels (arteries and arterioles), glycogen storage was not readily noted. These results suggests that the vascular energetics responsible for maintaining the pulmonary hypertensive state differ depending on the class of vessel. Also the muscular pulmonary arteries and arterioles which exhibited the greatest change in glycogen metabolism, may be primarily responsible for the maintenance of pulmonary hypertension.