Exps. showed that air at and higher than 3 atm. pressure exerts a narcotic effect on man with the characteristics of euphoria, retardment of the higher mental processes, and impaired neuromuscular co-ordination. At 4 atm. these changes can be counteracted by increased effort, but at 10 atm. they amount to stupefaction with greatly impaired muscular activity. The increased partial pressure of O does not account for these changes. It is inferred that the atmospheric N is the etiologic factor, and that it acts on the nervous system because of the high solubility coefficient of this gas in lipid substances compared with that in water.