The respiratory tolerance of the beta 1-adrenoceptor-selective blockers bisoprolol and metoprolol was investigated in eight male asthmatic patients. The interaction with the bronchodilatory effect of the beta 2-adrenoceptor-selective agonist terbutaline was studied to compare the beta 1-adrenoceptor selectivity of 10 and 20 mg bisoprolol and 100 mg metoprolol. The three beta-blocker doses caused a significant decrease in heart rate at rest. Systolic and diastolic blood pressure decreased significantly after administration of 20 mg bisoprolol and 100 mg metoprolol, but not after administration of 10 mg bisoprolol. Both beta-blockers caused an increase in bronchoconstriction, as measured by a significant fall in peak expiratory flow rate (PEFR). Only 10 mg bisoprolol caused a significant decrease in vital capacity (VC) and forced expiratory volume in 1 s (FEV1). Bronchoconstriction was rapidly reversed by inhalation of terbutaline in all patients. There were no differences between the responses of FEV1, VC, and PEFR to terbutaline with any of the beta-blockers, indicating a good degree of beta 1-adrenoceptor selectivity of 10 and 20 mg bisoprolol and 100 mg metoprolol in humans.