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.