What are the acute bronchopulmonary reactions which occur in smokers when 20 cigarettes are smoked over 10 hours during the day after a 12-hour interruption of smoking at night? Nineteen healthy chronic smokers with normal lung function aged from 22 to 39 years were investigated. The inhalative smoking characteristics were documented by alterations (p < 0.0005) of nicotine, cotinine and COHb in the blood. In body plethysmography, the subjects investigated showed a fall (p < 0.0025) of the airway resistances by 0.51 (cm H2O/l/s), a decrease (p < 0.005) of the trapped air and a rise (p < 0.01) of the thoracic gas volume by 0.58 (1) and (p < 0.01) of the specific conductance by 0.04 (cm H2O x s)-1 after smoking. The frequency-dependent compliance (40 breaths per minute) did not show any change. As a result of smoking, there was a fall (p < 0.025) of the arterial partial pressure of oxygen by 3.1 (mmHg) and carbon dioxide (p < 0.025) by 1.0 (mmHg) in ergometric exercise (women 75 Watts, men 100 Watts) as well as of the partial pressure of CO2 (p < 0.05) by 0.9 (mmHg) at rest. At the same time, the alveolo-arterial CO2 difference increased (p < 0.025) by 2.5 (mmHg). The results are interpreted as acute ventilation perfusion disorders which are reversible in the nocturnal resting phase.