1. Groups of young rats were kept in a hypoxic chamber or in air as control animals for 28 days.
2. Hypoxic and control animals were treated with either α-methyl-3,4-dihydroxyphenylalanine (α-methyldopa) or distilled water.
3. α-Methyldopa significantly reduced the increase in pulmonary artery pressure and right ventricular hypertrophy induced by chronic hypoxia and partially prevented the histological changes in the small pulmonary vessels.