Inhaled anticholinergic drugs are effective in the treatment of chronic obstructive pulmonary diseases (COPD), of acute asthma, and of some patients with nocturnal asthma. Tiotropium bromide (tiotropium) is a novel anticholinergic agent with a long duration of action and kinetic selectivity for M1- and M3-subtypes of muscarinic receptors. We investigated the duration of protection of a single dose of inhaled tiotropium against methacholine-induced bronchoconstriction in 12 male atopic asthmatic volunteers in a double-blind, placebo-controlled study. On four separate occasions 8 to 24 d apart, methacholine PC20 was measured serially for up to 48 h after placebo and after three doses of tiotropium (10, 40, and 80 microg). Each dose of tiotropium produced mild bronchodilatation as measured by an increase in FEV1 of between 5.5 and 11.1% from baseline, that was sustained for 24 h. There was significant dose-dependent protection against methacholine challenge at 2 h of 5.0 +/- 1.1, 7.1 +/- 0.5, and 7.9 +/- 0.7 (mean +/- SEM) doubling doses after 10, 40, and 80 microg respectively, and this persisted for 48 h. There were no adverse effects reported at any dose. The prolonged bronchodilator response and protection against methacholine challenge suggest that tiotropium may be useful in the treatment of COPD and nocturnal asthma and that once-daily dosing may be sufficient.