Adrenal cortical stimulation by adrenocorticotropic hormone (ACTH) in 16% gelatin and by corticotropin-zinc hydroxide, an aqueous repository preparation, was studied in six normal subjects. Forty units of corticotropin-zinc hydroxide produced adrenal stimulation similar to that of 80 units of ACTH in 16% gelatin. Doubling the dosage to 80 units of corticotropin-zinc hydroxide prolonged the action of this preparation up to two days in two of seven subjects tested. Plasma ACTH levels, produced by 40 and 80 units corticotropin-zinc hydroxide, were studied in one subject and showed a twofold rise with doubling of the unitage. This was associated with a 46% increase in steroidogenesis as measured by plasma 11-hydroxycorticosteroids. One may conclude from our observations that corticotropin-zinc hydroxide, a highly purified and stable preparation, is an effective adrenal cortical stimulant.