In ten normal young subjects four alveolar O2-concentration levels ranging from about 7% to 95% were obtained by breathing different concentrations of oxygen in nitrogen at a constant alveolar CO2-concentration of about 5%. Maximum expiratory flows at 60% TLC were measured at each level and corrected for the influence of the expired gas density on flow. The corrected maximum flows were not significantly different, which is taken as evidence of no effect of oxygen on tone of peripheral bronchi.