We wished to determine the independent effect of inspiratory flow rate (VI) on respiratory output in the awake state. Seven normal subjects were connected to a volume-cycled ventilator in the assist/control mode. While the tidal volume setting remained constant, inspiratory flow rate was increased in steps (3 to 4 min each) from 30 to 90 L/min and then back to 30 L/min. Flow pattern was square, and all breaths were subject-triggered. Spontaneous respiratory rate (f) increased in all subjects as VI increased. Mean (+/- SD) f at 30 and 90 L/min were 8.8 (+/- 1.7) and 14.1 (+/- 4.4) min-1, respectively (p less than 0.01). VE increased (10.1 +/- 1.9 at 30 L/min to 16.3 +/- 5.6 at 90 L/min p less than 0.01) and the percentage of end-tidal CO2 decreased (4.7 (+/- 0.5) at 30 L/min to 3.3 (+/- 0.7) at 90 L/min (p less than 0.01)). The response was graded and reversible. The change in f was nearly complete within the first two breaths after transition, and there was no systematic tendency for f to change beyond this early response. The rate of change in airway pressure prior to triggering (dp/dt) increased as VI increased (5.3 +/- 2.0 cm H2O/s at 30 L/min to 10.8 +/- 2.6 cm H2O/s at 90 L/min; p less than 0.01). We conclude that inspiratory flow rate exerts an excitatory effect on respiratory frequency and on the rate of rise of inspiratory activity in conscious humans. This may be relevant to the mechanism of ventilatory response to various respiratory stimuli and to the management of patients receiving mechanical ventilation.