Rats have been kept in a hypobaric chamber, and thus hypoxic, for up to 28 days in order to study the structural changes in the pulmonary arteries during the development of pulmonary hypertension. Rats were studied after 3, 5, 7, 10, 14, 21 and 28 days at a pressure of 380 mmHg. Right ventricular hypertrophy was demonstrated after 5 days in the hypoxic environment but increased up to 10 days. After pulmonary arterial injection microscopic counts of small arteries showed that vessels up to 200 mum external diameter were gradually "lost", reducing the ratio of arterial to alveolar number significantly by 14 days. No vestiges of these vessels were found with light microscopy. At the same time a gradual increase in arterial wall thickness was demonstrated and also progressive extension of muscle into smaller and more peripheral vessels than normal. In both these features maximum increase was reached by 14 days of exposure though changes were apparent after only 3. Similar changes have been found in patients with cystic fibrosis and chronic bronchitis using the same measuring techniques.