Respiratory mechanics during and following exposures to SO2 (36ppm) and NO2 (15ppm) for 10 minutes were compared in eight healthy male volunteers. In both instances, means of the pulmonary flow resistance significantly increased at the end of the exposure. No significant changes were observed in the other measures of respiratory mechanics. As to the responses of pulmonary flow resistance to SO2 and NO2 in each subject, all the subjects except one did not appear to show a peculiar response to only either gas. Additional groups of subjects were exposed to SO2 and NO2 with and without pretreatment with atropine sulfate. The atropine inhibited markedly the increase in the pulmonary flow resistance during the exposure to SO2, but not during the exposure to NO2. This result suggests as regards the causes of airway narrowing that the reflex broncho-constriction have played a role in the exposure to SO2 and that other mechanisms have also been involved in the case of NO2.