The effect of background loading on magnitude estimation of added elastic and resistive inspiratory loads was determined. An analogous study involving estimation of the heaviness of weights in the hand was also performed. Perceptual performance was assessed using Stevens' power law psi = k phi n, where psi is the subjective magnitude, phi is the peak mouth pressure generated with an inspiratory load or the weight of the load in grams for the heaviness estimation, and the exponent n characterizes perceptual performance. The value of n was determined for the control and background conditions for each study. The results for both inspiratory loading studies and the heaviness estimation experiment indicate that background loading is associated with a significant increase in the exponent for magnitude estimation (P less than 0.05). Adjustment of the stimulus scale by subtracting the difference in peak mouth pressures generated during resting breathing between control and background-loaded conditions for the inspiratory loading studies, or the weight of the background load in the heaviness estimation experiment, converted the exponents obtained under background-loaded conditions to values that were not significantly different from those for control (P greater than 0.05). These results are consistent with the theory suggesting that an increase in detection threshold, produced by the background load, is responsible for the increase in exponent for magnitude estimation.