During mechanical ventilation, changes in inspiratory flow and tidal volume (VT) have been shown to alter respiratory frequency (f ). However, the changes in flow and VT have been accompanied by alteration in ventilator inspiratory time (TI,(vent)), and it is not clear which variable is the primary determinant. To address this issue, we employed four protocols in 15 healthy volunteers receiving assist-control ventilation. When VT was fixed and flow was delivered at 30, 60, and 90 L/min, f increased as a function of the increase in flow and the decrease in TI,(vent). When flow was held constant and VT was changed among 0.5, 1.0, and 1.5 L, f increased as a function of the decreases in VT and TI,(vent). When flow was increased from 60 to 90 L/min and these changes were balanced with VT settings of 1.0 and 1.5 L to maintain a constant TI,(vent), f did not change. When flow and VT were held constant and TI,(vent) was varied by the application of inspiratory pauses (0 to 2 s), f decreased as a function of the increase in TI,(vent) (p < 0.001). In conclusion, the imposed ventilator inspiratory time during mechanical ventilation can determine f independently of delivered inspiratory flow and VT.