The intravenous infusion of 20 mg of adenosine 3′,5′-monophosphate (3 ′,5 ′-AMP), N6-2 ′-O-dibutyryl-3 ′,5 ′-adenosine monophosphate (dibutyryl 3′,5′-AMP) or 2′-O-acetyl-3′,5′-adenosine monophosphate (monoacetyl 3′,5′-AMP) for 12 min caused a significant increase in adrenal corticosterone secretion in hypophysectomized rats, while other mononucleotides had no effect. The increase in the corticosterone content of adrenal venous blood was greatest with dibutyryl 3′,5′-AMP, intermediate with 3′,5′- AMP and least with monoacetyl 3′,5′-AMP. The effect of 20 mg of dibutyryl 3′,5′-AMP was almost the same as that of 1 mU of ACTH. However, the administration of 3′,5′-AMP and its derivatives significantly decreased adrenal blood flow, while ACTH increased it to some extent. The intravenous infusion of 3′,5′-AMP and its dibutyryl derivative caused a significant increase in plasma corticosterone in the systemic blood of hypophysectomized rats and of dexamethasoneblocked rats. In intact rats, however, only dibutyryl 3′,5′-AMP significantly affected plasma corticosterone. (Endocrinology76: 933, 1965)