1. The initial rate of change of pressure at the mouth (d P /d t ) during a brief occlusion of the airways at the beginning of inspiration has been estimated in nine healthy subjects at rest, during exercise and during the first 2 min of recovery. Exercise was carried out with progressively increasing loads to the maximum tolerated (progressive exercise) and also for a period of 6 min at a constant load of 60% of the maximum (steady-state exercise). 2. A highly significant linear relationship was found between work loads and d P /d t during progressive exercise in all our subjects. 3. A highly significant linear relationship was found between ventilation and d P /d t in both forms of exercise, but the slope of the regression line was steeper during progressive than during steady-state exercise in six out of nine subjects. 4. The pattern of breathing ( V T , f, V T / T insp. , T insp. / T tot. ) did not account for the difference in the relationship between d P /d t and ventilation during the two forms of exercise. 5. These results are in agreement with the hypothesis that d P /d t is an index of central inspiratory drive.
1. Progressive exercise and circulatory studies are reported on a selected group of patients with hypoxaemia and secondary polycythaemia before and after therapeutic decrease of packed cell volume. 2. A significant increase in exercise tolerance was demonstrated in a group of seven patients who claimed subjective benefit from the treatment. 3. No important circulatory changes were detected at rest after treatment. 4. During steady exercise mean pulmonary artery pressure at any given cardiac out-put was lower after treatment suggesting a fall in pulmonary vascular resistance. 5. During steady exercise oxygen consumption was increased in five of six patients subjectively improved by the treatment but no consistent changes in arterial lactate concentration or lactate/pyruvate ratio were observed.