1. The intensity of breathlessness was measured during exercise in nine normal subjects using a modified Borg scale to examine the effect of prior experience of breathlessness on subsequent estimates of breathlessness.
2. Each subject performed four exercise tests, each of which consisted of two identical runs of workload incrementation (run 1 and run 2). An inspiratory resistive load of 3.8 cmH2O s−1 l−1 was applied during the appropriate run of the exercise test to examine the effect of (a) prior experience of ‘loaded’ breathing on breathlessness estimation during ‘unloaded’ breathing, and (b) prior experience of ‘unloaded’ breathing on breathlessness estimation during ‘loaded’ breathing. Run 1 was the conditioning run; run 2 was the run in which the effect of conditioning was measured.
3. There was a good correlation between breathlessness and minute ventilation during both unloaded’ breathing (median r = 0.93) and ‘loaded’ breathing (median r = 0.95).
4. The slope of the Borg score/minute ventilation relationship was greater during ‘loaded’ breathing than during ‘unloaded’ breathing (P < 0.01). There was no difference in mean Borg score between ‘unloaded’ and ‘loaded’ breathing.
5. After a period of ‘loaded’ breathing during run 1, estimated breathlessness was significantly reduced during ensuing ‘unloaded’ breathing in run 2 (P < 0.01) compared with the exercise test in which ‘unloaded’ breathing was experienced throughout both run 1 and run 2.
6. After a period of ‘unloaded’ breathing in run 1, estimated breathlessness was significantly increased during ensuing ‘loaded’ breathing in run 2 (P < 0.01) compared with the exercise test in which the inspiratory load had already been experienced in run 1.
7. Changes in the pattern of breathing (inspiratory time, expiratory time, total breath duration, inspiration time/total breath duration ratio and tidal volume) were not consistent with the changes in breathlessness.
8. We suggest that perception of breathlessness may be influenced by a subject's immediate prior experience of an altered relationship between breathlessness and ventilation.