The acute broncholytic efficacy and the protective anticonstrictive effectiveness of a new beta 2-receptor stimulator, Reproterol, was tested in 16 patients with the acetylcholine (ACH) provocation test. Plethysmographic airway resistance determinations (Raw) and flow volume curves (VEmax50%/FVC) were used as assessment parameters. All patients proved susceptible to ACH provocation, in as much they demonstrated a significant rise in the Raw from 2.46 to 11.53 cm H2O/1/sec and a significant decline of the VEmax50%/FVC from 884 to 565 (ml/sec/1) following ACH administration. With Reproterol inhalation (2 puffs at 500 mu each), almost complete elimination of ACH-induced bronchial constriction was achieved. The ACH provocation with the same patients the following day, however, was almost absent 60 min after oral ingestion of 20 mg of Reproterol tablets. Dose-efficacy studies in 7 volunteers from the same group revealed a subjectively better tolerance of the 10-mg tablets, but with a correspondingly lower antibronchia constrictive efficacy.