The bronchoconstrictor effect of propranolol aerosol was investigated on guinea-pigs and on 5 patients with bronchial asthma. 2. In guinea-pigs a 12% propranolol aerosol regulare caused severe dyspnoea similar to that observed in the anaphylactic microshock. 3. The bronchial obstruction present in asthmatics became more pronounced after inhaling a 1% or 5% propranolol aerosol. 4. In guinea-pigs and in asthmatics the propranolol effect was significantly antagonized by atropine and by hexamethonium; guanethidine and mepyramine had no effect.