Adenyl cyclase in bovine adrenal cortex was distributed in all particulate fractions obtained by differential centrifugation; factors in the 105,000 g supernate markedly inhibited cyclase activity. A major fraction of the total enzyme is associated with mitochondria. In all membranous fractions, the enzyme was stimulated by NaF and ACTH, but not by MSH, thyrotropin, glucagon or vasopressin; epinephrine variably stimulated cyclase to a small extent relative to ACTH. © 1969.