The effects of tobacco smoke on the central and peripheral airways were assessed in humans and rats by direct and indirect methods. In both species tobacco smoke affected the central and peripheral airways. In humans there were apparent decreases in the 1-second forced expiratory volume, peak expiratory flow rate and significant increases in closing volume and closing capacity (P < 0.001). In rats significant changes were seen in specific airway resistance from the 6th week of exposure onwards. Similarly, airway luminal diameter decreased markedly in tobacco-exposed animals to subthreshold concentrations of acetylcholine (10-8M), This decrease was also exposure time dependent. The increased responsiveness of the respiratory system has been attributed to inter alia: (1) increased vagal activity; (2) increased mucus production leading to decreased airway lumen; (3) mucosal swelling due to changed ionic constellation; (4) disturbance of the lungs’ defense mechanism; (5) imbalance in and easy accessibility to the adrenoceptor