For over 30 years, P-adrenoceptor agonists have been used routinely in the treatment of bronchial asthma. The therapeutic use of these compounds is derived from their ability to prevent and reverse bronchoconstriction. A major theoretical advantage of the P-adrenoceptor agonists is that they act as functional antagonists to produce bronchorelaxation regardless of the contractile agonist used to induce tone. This universal activity is important since it is likely that the bronchoconstriction associated with asthma is produced by the concerted actions of several mediators acting through distinct classes of receptors. Consequently, the molecular mechanisms by which l3-adrenoceptor agonists relax airway smooth muscle have received considerable attention.