1 Ten patients suffering from extrinsic bronchial asthma were examined. 2 In the pretreatment period, the histamine concentration required to produce a 20% decrease of forced expiratory volume in one second (FEV1) was 7.71 mg/ml. 3 Histamine provocation was repeated 2 h after administration of placebo, propranolol 40 mg, pindolol 5 mg or metoprolol 50 mg. After pindolol and metoprolol the histamine concentrations were slightly, but not significantly, lower than after placebo. After propranolol only 3.16 mg/ml histamine was necessary to reduce FEV1 by 20%. 4 This indicates that in equipotent cardiac beta-adrenoceptor blocking doses, in contrast to propranolol, neither pindolol nor metoprolol increased sensitivity to the bronchoconstrictor effects of inhaled histamine.