The onset of the bronchodilating effect of formoterol (12 microg by Turbuhaler) was compared with that of salbutamol (50 microg by Turbuhaler), salmeterol (50 microg by Diskhaler) and placebo in methacholine-induced severe bronchoconstriction. Seventeen subjects with mild-to-moderate asthma completed this randomized, double blind, cross-over, double-dummy study. On four study days, baseline forced expiratory volume in one second (FEV1) was recorded and the subjects were challenged with methacholine until FEV1 fell by at least 30%. Immediately thereafter, the study drugs were inhaled and lung function was assessed for 60 min. The geometric mean time for FEV1 to return to 85% of baseline was 7.2 min with formoterol, 6.5 min with salbutamol, 14.1 min with salmeterol and 34.7 min with placebo (p=0.0001, overall ANOVA). The difference between formoterol and salmeterol was statistically significant (p=0.01); there was no difference between formoterol and salbutamol (p=0.69). In conclusion, formoterol reversed methacholine-induced severe bronchoconstriction as rapidly as salbutamol and more rapidly than salmeterol. Classifying beta2-agonists as "fast"- and "slow"- acting may be supplemental to "short"- and "long"-acting.