The cortisol level in the serum of mice, which had been exposed to the influence of negatively or positively ionized air for 30 minutes, was determined by means of competitive protein fixation. Under the influence of both ion moieties, the cortisol level rose to twice the physiological norm (p less than 0,001 in both cases). - Theoretically the inhaled air ions might exert influence on the regulatory cycle hypothalamus - hypophysis - ACTH - adrenal cortex both neurogenically and directly via the circulatory system, thereby changing the cortisol level. For this reason, the determination of ACTH, as well as of cortisol after hypophysis exstirpation, must be regarded as a prerequisite for further investigations.