Using open-magnitude scaling, we compared the perceived magnitude of externally added resistive and elastic loads to breathing in normal subjects with that perceived when the background load (i.e., the minimum load of the circuit) was increased by the addition of either resistive or elastic loads of increasing magnitude. The study was carried out over four experimental sessions. After a control experiment (no added background load), the background load was increased by the addition of either a resistive or an elastic load for a duration of 3 min. The perceived magnitude of a further series of loads, proportionately increased, was then ascertained. This sequence was then repeated after a further increase in background. The results showed that the perceived magnitude of the load was closely related [mean r = 0.96 +/- 0.01 (SE)] to the magnitude of the physical stimulus expressed as the peak inspiratory pressure by a power function relationship in keeping with Stevens' law. After the increases in background resistance or elastance, there were no significant differences in either exponents or intercepts compared with basal conditions. There was no significant difference in the perceived magnitude of the loads after adaptation. At the smallest load, the perceived magnitude was less than expected from the control experiment. However, this reduction did not reach statistical significance. In the special senses, moderate-to-large stimuli show little change after adaptation, whereas small stimuli are reduced. Although not conclusive, we suggest that the relationship is similar with loaded breathing.