1. Pulmonary arterial mean pressure, cardiac output and other variables have been measured at rest and during exercise breathing air and oxygen in eight selected working coalminers with pneumoconiosis; four of the subjects had no respiratory symptoms and four had chronic cough and sputum but a well maintained ventilatory capacity. 2. The relationship of cardiac output to cardiac frequency during exercise was used to estimate cardiac output by interpolation with a residual standard deviation of 0·28 1/min. Used in this way the curves provided additional points for construction of mean pressure/flow curves. 3. In the subjects without respiratory symptoms the pulmonary arterial mean pressure when breathing air was similar to that in normal subjects. In the subjects with respiratory symptoms the pressure was normal at rest but consistently increased on exercise, although the arterial oxygen tension was relatively normal. The findings are interpreted as evidence for an increase in pulmonary vasomotor tone in patients with clinically mild chronic bronchitis. 4. Breathing oxygen during exercise led to a reduction in pulmonary arterial mean pressure in all subjects but not to a change in slope of the mean pressure/flow curve; a similar displacement of the curve was observed in some bronchitic subjects on repetition of the exercise breathing air. The response may be due to the small pulmonary vessels acting as a vascular sluice rather than to a change in calibre of the resistance vessels.