We have studied the perceived magnitude (psi) of a range of externally added resistive (delta R) and elastic (delta E) loads to breathing while varying the flow rate, tidal volume, and inspiratory duration. The sensory magnitude of an added resistive load increased with both the inspiratory flow rate (P less than 0.001) and the magnitude of the added resistance (P less than 0.01). Similarly, the perceived magnitude of an added elastic load increased with both the tidal volume (P less than 0.001) and the magnitude of the added elastance (P less than 0.01). When inspiratory duration was not controlled, a single relationship was found to hold for both types of load when analyzed as a function of inspiratory airway pressure (P): psi = K . P1.15 (r = 0.94 for delta R); psi = K . p1.10 (r = 0.98 for delta E). When inspiratory duration was controlled, the perceived magnitude of both resistive and elastic loads also increased as the inspiratory duration increased (P less than 0.05). Again, the sensory magnitude was closely related to the airway pressure; but for any given airway pressure the perceptual magnitude also increased as inspiratory duration increased: psi = K . t0.6 (for delta R); psi = K . t0.52 (for delta E). When resistive, elastic, and mixed resistive and elastic loads were presented while keeping inspiratory duration constant, the sensory magnitude was directly related to the airway pressure (r = 0.99) and there was no discernible difference between the types of load. The results suggest that the perceived magnitude of added loads to breathing is directly related to the inspiratory muscle force and its duration (psi = K . P1.3 . t0.56) and indirectly to the added load.