1. The initial rate of change of pressure at the mouth (dP/dt) 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 dP/dt during progressive exercise in all our subjects.

3. A highly significant linear relationship was found between ventilation and dP/dt 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 (VT, f, VT/Tinsp., Tinsp./Ttot.) did not account for the difference in the relationship between dP/dt and ventilation during the two forms of exercise.

5. These results are in agreement with the hypothesis that dP/dt is an index of central inspiratory drive.

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