Abstract: The adrenocortical hormone activities of △⁶-6-methyl cortisone acetate (△⁶), synthesized by the Institute of Organic Chemisty of the Academia Sinica, are studied and compared with those of cortisone. When adrenalectomized rats, weighing 100—200grams were injected subcutaneously with a dose of 1 mg of △⁶ once daily for 4 days, the developments of granuloma on subcutaneous implantation of cotton pellets and of egg-white-induced oedema of the hind paws were inhibited markedly. Involution of the thymus of these rats was also noted. The potency of △⁶ in these respects was similar to that of cortisone. In adrenalectomized rats and mice, the liver glycogen deposition activity of △⁶ was 4—5 times as potent as cortisone. When rats were pre-treated with △⁶ at a dosage of 0.75 mg/rat, the emotional stress activated secretion of pituitary ACTH was markedly inhibited. With respect to water and electrolyte regulation, △⁶ differs from cortisone and deoxycorticosterone in that, in stead of possessing water and sodium retaining actions, it induces diuresis in adrenalectomized rats. From these findings it would appear, therefore, that the adrenocortical hormone activities of △⁶ resemble those of 6α-methyl prednisolone, triamcinolone, and dexamethasone.