We examined the effect of inspiratory flow rate (IFR) on respiratory sensation during mechanical ventilation in 10 normal subjects. We adjusted the ventilator tidal volume (VT), frequency, and IFR until subjects indicated that they were maximally comfortable ("comfort IFR"). Subjects then rated breathing discomfort on a visual analog scale (VAS) while IFR was varied among four levels: 70%, 100%, 200%, and 300% of the comfort IFR. When compared with VAS ratings at the comfort IFR (4.4 +/- 1.2, mean +/- SEM), VAS ratings were significantly greater at the lowest (i.e., 70% comfort; 12.1 +/- 2.1) and highest (300% comfort; 8.2 +/- 0.9) IFR; there was no difference in ratings between the comfort IFR and 200% comfort IFR. At the lowest IFR, the breathing discomfort arose in the chest and had an air hunger-like quality; at high IFR, the discomfort arose in the upper airway. In the second portion of the study, subjects used open magnitude estimation to rate breaths of five different sizes at three different IFR (70%, 100%, and 200% of comfort rate). Neither the exponent nor intercept for VT perception differed among the three IFR. Our results demonstrate that although IFR does not alter magnitude estimation of breath size, deviations of IFR from that desired by the subject may greatly affect respiratory comfort.