Tiotropium is a long-acting anticholinergic drug. Studies with cloned human muscarinic receptors show that tiotropium binds equally well to M(1), M(2), and M(3) receptors. However, it dissociates very slowly from M(1) and M(3) receptors compared with ipratropium, and more rapidly from M(2) receptors. Binding studies with [(3)H]tiotropium in human lung show that it is approximately 10-fold more potent than ipratropium. In vitro, tiotropium has a potent inhibitory effect against cholinergic nerve-induced contraction of airways. It dissociates extremely slowly, compared with the dissociation of atropine and ipratropium. Clinical studies with single doses of inhaled tiotropium confirm that it is a potent and long-lasting bronchodilator. Furthermore, it protects against cholinergic bronchoconstriction for > 24 h. Pharmacokinetic studies show that little of the inhaled drug is absorbed, thus predicting a high margin of safety.