The effect ofsmoking a cigarette on bronchial responsiveness was studied in healthy nonsmokers. Twenty two subjects performed a methacholine inhalation test before and after smoking a single cigarette. Ten of the subjects took part in a further study in which propranolol was inhaled before the smoking challenge to diminish the baseline adrenergic tone of the airway. After they had smoked a single filtered or non-filtered cigarette the indices of bronchial responsiveness (the cumulative dose of methacholine starting a decrease in the reciprocal of resistance, Grs (Dmin), and the cumulative dose causing a 35% drop in the Grs (PD35Grs)) did not change significantly. With the inhalation of propranolol mean (SD) log Dmin decreased from 1-37 (0 44) units to 0 74 (0 57) (p < 0-01) and log PD35Grs from 193 (0-38) to 1l51 (0 38) (p < 0-01). Smoking a single cigarette after the inhalation of propranolol did not, however, cause any further change in bronchial responsiveness. This study suggests that smoking a single filtered or non-filtered cigarette does not change bronchial responsiveness in non-smokers, and that changes in adrenergic tone of the airway do not modify the effect of smoking a single cigarette on bronchial responsiveness.Bronchial responsiveness to non-specific stimuli such as histamine, methacholine, and acetylcholine is increased in people with bronchial asthma and in those with chronic obstructive lung disease.' Even in normal individuals conditions such as a viral infection of the respiratory tract,2 ozone exposure,3 and airway cooling4 influence bronchial responsiveness. What remains controversial is the effect of chronic smoking on bronchial responsiveness.'Smoking a cigarette induces bronchoconstriction in small and large airways.9"' In guinea pigs inhalation of cigarette smoke increased bronchial responsiveness." It is not known, however, whether smoking one cigarette affects bronchial responsiveness in man. In the present study we have tested the hypothesis that the smoking of a single cigarette increases bronchial responsiveness.