The measurement of forced expiratory volume (FEVS) was shown to be a valuable parameter in the study of various bronchodilators in coal-miners suffering from bronchospastic conditions. The study was performed in 124 coal miners. The effects of a new selective betamimetic bronchodilator, Th 1165 a, in the form of a metered aerosol, tablets and ampoules, were compared with those of isoprenaline (Aleudrin), orciprenaline (Alupent), adrenaline (epinephrine) and atropine methyl nitrate. The drugs were compared as to intensity and duration of action, cardiac action being monitored continuously with an E.C.G. The results were statistically evaluated and are presented as tables and graphs. Th 1165 a was shown to be superior to all the betamimetic bronchodilators tested with regard to its selective action on the bronchi, its duration and intensity of action as well as lack of side effects and absence of tolerance following long-term administration.