We looked at the perception of breathlessness associated with bronchoconstriction during early (EAR) and late (LAR) asthmatic responses to inhaled antigens and its correlation with the rate of fall of expiratory flows. Twenty-eight asthmatics were studied (12 male, 16 female, 18 to 38 yr of age). Breathlessness was evaluated on a modified Borg scale (0 to 10) before each FEV1 measurement, obtained at regular intervals for as long as 8 h after the antigen challenge. The rate of the fall in FEV1 was calculated from the maximal percent fall obtained during the EAR or the LAR and the time lapse from the onset of the reaction to the maximal fall in FEV1. Ten subjects had an isolated early and 18 had a dual asthmatic response after allergen challenge. Dual responders perceived a similar percent fall in FEV1 (mean percent fall: EAR, 20.7 +/- 1.6%; LAR, 21.5 +/- 1.6%) more intensely during the EAR than during the LAR, with median Borg scores (range) of, respectively, 2.0 (0.5 to 4) and 0.5 (0 to 3.0) (n = 18, p < 0.001). The median rate of the fall in FEV1 was: EAR, 2.09 (0.77 to 6.6) %/min; LAR, 0.11 (0.05 to 0.36) %/min (n = 18, p < 0.001). The rate of the fall in FEV1 during either EAR or LAR was strongly correlated with the Borg scores; the slower the fall, the weaker the perception (p < 0.001). We conclude that LAR are poorly perceived compared with EAR and that this may be due to the temporal adaptation to the slow and progressive bronchoconstriction in LAR.