We compared resting breathing patterns in twelve normal humans before and after administration of aerosols of normal saline and 4% lidocaine. Experiments were performed in random order on two consecutive days in single blind fashion. At the time of the experiments, neither the subjects nor the people who analyzed the results knew the purpose of the study. Neither saline nor lidocaine aerosols produced changes in FEV1.0 or end-tidal PCO2, or, in the six subjects in whom measurements were made, changes in functional residual capacity or airway resistance. Aerosols of normal saline produced no change of ventilation (VI), breathing frequency (f), tidal volume (VT), duration of inspiration (Ti), mean inspiratory flow (VT/Ti) or the fraction of the breathing cycle devoted to inspiration (Ti/Tt). Lidocaine aerosols, on the other hand, were associated with a decrease in f (P less than 0.01), an increase in Ti (P less than 0.01), an increase in VT (P less than 0.02) and a decrease in VI (P less than 0.05), while VT/Ti and Ti/Tt were unchanged. We conclude that lidocaine aerosols, by compromising the function of vagal airway receptors, altered breathing pattern in normal resting humans, indicating that these receptors play a role in regulating breathing pattern under these circumstances. Though the changes we observed are most simply explained as being due to decreased stretch receptor activity, we cannot be certain that such was the case.