In order to study the relations between maximal oxygen consumption during exercise (VO2max) and the pulmonary circulation when pulmonary artery pressure (PPA) and pulmonary vascular resistance (PVR) are normal or close to normal, we evaluated twenty-three patients with chronic bronchitis, in stable clinical condition. All these patients performed a progressive exercise test on a bicycle ergometer until exhaustion (VO2max); they were also subjected to right heart catheterization at rest and during light exercise at constant load. In these patients with moderate functional impairment, we observed on average a significant, although weak, correlation (r = -0.52, P less than 0.01) between pulmonary vascular resistance at rest and VO2max. When VO2max exceeded 2 1 min-1, PVR was below 210 dyn s cm-5. From the heart rate during maximal exercise we computed the oxygen pulse (O2Pmax). In eight patients O2Pmax was reduced (less than or equal to 11 ml) with maximal heart rate close to its predicted value; in seven of these subjects PVR was abnormal and Ppa at rest was at least 20 mmHg. Thus it seems that in chronic obstructive bronchitis mild disturbances in pulmonary circulation may be a contributory factor in the limitation of exercise performance, and a maximal exercise test may help to detect pulmonary hypertension in these patients.