1. The ability of normal subjects to estimate the magnitude of resistive loads and to detect resistive loads was determined using standard psychophysical techniques. The experiments were repeated during maintained partial neuromuscular blockade with D-tubocurarine. 2. During curarization the ability to detect the presence of a small inspiratory resistive load was not significantly impaired. This finding is consistent with the view that changes in the usual breath-by-breath relation between pressure and flow mediate detection. 3. The size of resistive loads was over-estimated during partial curarization. Subjects thus placed more reliance on sensing the increased motor command required during curarization than on alternative peripheral signals related to pressure, flow or resistance. 4. The exponent for the power function relating the perceived magnitude of a resistive load to its actual size (Stevens, 1957) was reduced during partial neuromuscular blockade. 5. These results suggest that detection of resistive loads depends upon sensing apparent information arising from an unexpected pressure-flow disturbance but that estimation of the size of an added load depends, in part, on sensing the outgoing motor command.