This study investigated the relationship between resistive load magnitude, load magnitude estimation, and the respiratory-related evoked potential. In Part 1, 10 healthy subjects estimated the magnitude of five inspiratory resistive loads. Two subjects were shown to have a markedly reduced slope of the magnitude estimation-resistive load relationship and were suggested to be "poor perceivers" of respiratory stimuli. In Part 2, evoked potentials were recorded from the same 10 subjects using the same resistive loads as Part 1. A log-log plot of the group averaged P1 amplitudes showed a linear relationship with resistive load. Aberrant P3 components were seen in the 2 poor perceiving subjects and one of the 2 showed no late response. In the other 8 subjects, P3 varied as a function of resistive load, being augmented to larger loads. These results provide evidence that P3 may be a key index of the perception of respiratory sensitivity and effort.