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407system was present in both fascicular and reticular zones. The comparatively low concentrations in the glomerulosa were due a t least in part to1 a relative lack of specific dehydrogenase activity. One can not rule out by this method the possibility that TPN may also be low in the glomerulosa, because T P N is added in reaction I. If as has been suggested one of the roles played by this enzyme system in the cortex is to act as a TPNH generator, providing this reductant for important hydroxylations of the steroid molecule? it is reasonaible that it be most active in the fascicular and reticular zones. These are the areas which seem to react most strikingly in situations of acute and chronic stress and following ACTH administration. lbrphologic and histochemical criteria of response to stress including ascorbic acid depletion, lipid and cholesterol depletion and hypertrophy all appear to involve the reticular and fascicular zones( 6). It has not been demonstrated that the glomerulosa participates actively in these adjustments, a t least in the early stages. This zone appears to react demonstrably in a specific fashion to alteration in eleclrolyte concentration. In the light of our present knowledge it would seem that the faszicular and reticular zones are the areas of the cortex where increased synthesis of steroids occurs in response to acute and chronic stress, and it is here that one would suspect a requirement for iincreased T P N H and potential T P N H generator sys tems. Summary. A histochemical technic was used to localize the activity of glucose-6-phosphate dehydrogenase system in zones of the adrenal cortex. I t was very active in the fascicular and reticular zones. Activity in the glomerulosa was comparatively low. Since this enzyme system may be indirectly influencedl by ACTH and is a potential provider of TPNH, a reductant important in steroid synthesis, demonstration of its predominance in a zone where active steroid synthesis is presumeld to occur is reasonable and of considerable interest.