The acuity of resistive load detection (RLD) has been tested by comparing threshold detection (delta R50) under control conditions with that obtained when loads were applied at different times in inspiration, with different inspiratory flows, at different lung volumes, and with different background loads. For loads applied suddenly during established inspiratory flow, the mean delta R50 was 0.94 +/- 0.04 (+/- SEE) cmH2O . 1(-1) . s compared with 0.42 +/- 0.13 cmH2O . 1(-1) . s (P < 0.01) for loads applied before inspiration, suggesting that information generated early in the breath is important and the RLD is not subserved by a simple peripheral proprioceptive mechanism. Rapid voluntary inspiration through nonlinear resistances did not improve RLD (mean delta R50 0.86 cmH2O . 1(-1) . s compared with a control of 0.56 cmH2O . 1(-1) . s; P < 0.05) despite the much greater pressures and resistances at high flow rates, further suggesting that detection of a resistive load occurs early in inspiration. Alteration of the background load by breathing at increased lung volume (1 liter above FRC) or by breathing against a standing pressure of 1 or 3 cmH2O (i.e., pressure independent of volume or flow) did not significantly reduce RLD. This suggests that the change in pressure due to the added load, expressed as a fraction of the total driving pressure, cannot subserve RLD. Our results suggest that external resistive load detection is subserved by the relation between pressure and flow over the early part of inspiration.