The ability of normal subjects to detect the addition of external resistive loads was determined both at rest and when ventilation was increased to more than 30 L/min by CO2 administration or exercise. Tidal volume, flow, and mouth and transpulmonary pressures were recorded during resistive load discrimination using standard psychophysical techniques. The mean resistance detection threshold was 0.40 +/- 0.06 cm H2O/L/s at rest, 0.36 cm H2O/L/s during administration of CO2, and 0.44 cm H2O/L/s with exercise. The Weber fraction was 0.28 +/- 0.04 at rest, 0.29 +/- 0.08 with CO2, and 0.34 +/- 0.04 with exercise. The lack of change in the ability to detect resistive loads when ventilation was increased was surprising because changes in both pressure and flow were tripled. The findings were supportive of the hypothesis that resistance or something like it is perceived, rather than change in flow or pressure.