Abstract
No significant differences were detected in the quantitative histological distribution of the succinic dehydrogenase system in adrenals from adult male albino rats sacrificed at midnight and at noon. Peak activities were found in the reticularis. Subcutaneous injections of 25 mg. ACTH per kg. 3 hours before sacrifice had little if any effect on the enzyme activity per unit protein-nitrogen in the various histological zones of the rat adrenal. However, daily subcutaneous injections of 7 mg. cortisone acetate per kg. for 2 weeks before sacrifice elicited some increase in all zones. Six days after hypophysectomy lowered activity was found only in the reticular and in the mixed reticular and medullary zones.