The intensity of cortisol metabolism was studied during perfusion of the liver in situ with solutions containing different concentrations of cortisol. Under these circumstances metabolism was shown to take place chiefly in the direction of cortisone formation. With an increase in the cortisol concentration in the perfusion fluid the intensity of its metabolism in the liver tissue also increased. During anaphylactic changes in the liver tissue the intensity of conversion of cortisol into cortisone was reduced, indicating a disturbance of the oxidation of cortisol. In experimental allergic encephalomyelitis the intensity of cortisol metabolism also was reduced.