1. Recently, a patch formulation of tulobuterol, a beta-adrenoceptor (AR) agonist, has been developed using a transdermal delivery system. The present study was designed to determine whether beta-AR function and asthma control were affected by the sustained-released beta-AR agonist. 2. Tulobuterol (2 mg) was applied daily for 8 weeks to seven patients with bronchial asthma in whom the morning dip in the peak expiratory flow (PEF) rate developed even though inhaled glucocorticoids were being taken. After treatment with tulobuterol, the early morning reduction in PEF was suppressed and PEF values were increased from 367 +/- 35 to 439 +/- 38 L/min (P < 0.05). The rescue use of inhaled beta-AR agonists was decreased from 6.9 +/- 2.0 to 1.0 +/- 0.7 puffs/week (P < 0.01). Symptom scores also decreased from 8.3 +/- 3.4 to 2.1 +/- 1.4 score/week (P < 0.01). 3. Next, we sought to examine the effects of exposure to tulobuterol on beta-AR function in guinea-pig tracheal smooth muscle. After exposure of tissues to tulobuterol (0.01-10 micro mol/L) for 45 min, the inhibitory effects of tulobuterol on methacholine-induced contractions were attenuated in a concentration-dependent manner. However, the inhibitory effects of tulobuterol were not affected after exposure to 0.01 micro mol/L tulobuterol (a concentration greater than serum levels in clinical use). In contrast, the inhibitory effects of procaterol were not affected after exposure to tulobuterol under the same experimental conditions. 4. These results indicate that the combination of sustained-released tulobuterol with inhaled glucocorticoid therapy is beneficial to patients with bronchial asthma who suffer from symptoms induced by the morning dip in PEF. Moreover, chronic exposure to lower concentrations of tulobuterol does not lead to desensitization of beta-AR in airway smooth muscle.