1. We have examined the effects of aminophylline on the respiration and pulmonary circulation of eleven patients with chronic bronchitis and six patients with peripheral bronchial carcinoma; the latter were free from bronchial obstruction at the time of study.
2. Aminophylline caused an increase in total and alveolar ventilation and a decrease in arterial carbon dioxide tension. Lung diffusing capacity was unaltered in subjects with marked respiratory insufficiency but increased slightly in an additional group of less severely affected patients, and in the control subjects.
3. Mean pulmonary arterial pressure decreased significantly in the patients with chronic bronchitis but not those with lung cancer. A positive correlation was observed between the level of pulmonary arterial pressure during the control period and the decrease after aminophylline.
4. For the group as a whole there was no significant change in cardiac output or arterial oxygen saturation or tension. However, in those subjects in whom the cardiac output was increased, the arterial blood oxygen was reduced despite an increase in alveolar ventilation. The data are interpreted as evidence for a disproportionate part of the increase in cardiac output being directed to poorly ventilated areas of the lung.