The medial thickness of the pulmonary trunk and of the elastic and muscular pulmonary arteries was measured in 4 mountain-viscachas (Lagidium peruanum) born and living at La Raya (4 200 m) in the Peruvian Andes. The ventricles of the heart were weighed separately. In spite of living in a state of chronic hypoxia the viscacha has a thin-walled pulmonary vasculature in contrast to that of Quechua Indians and domesticated animals in the mountains. This is thought to be an expression of the fact that whereas man and cattle are acclimatized to high altitude, indigenous mountain animals are adapted to these conditions. During the course of evolution at high altitude the mountain-viscacha has lost the property of pulmonary vasoconstriction, finding it advantageous to avoid the harmful effects of pulmonary hypertension at the expense of the benefits of ventilation: perfusion homogeneity. © 1981.