Adenylate cyclase was solubilized in a relatively stable form from rat adrenal membranes. The solubilized enzyme elutes on a column of Sepharose 4B as a distinct peak with a higher molecular weight than the soluble fractions which bind 125I-ACTH. Both the soluble and membrane bound enzymes are activated by NaF and Gpp(NH)p, and both have similar affinities for MgATP. While the membrane bound enzyme is activated similarly by either Mg2+ or Mn2+, the soluble enzyme is more fully activated by Mn2+. Pretreatment of adrenal membranes with NaF or Gpp(NH)p before the addition of detergent enhances recovery of soluble enzyme activity, while recovery of activity in the unsolubilized membrane pellet is unchanged. In contrast, addition of ACTH prevents solubilization of the enzyme and greatly increases its recovery in the pellet. This observation is consistent with the theory that the action of the hormone on a receptor subunit leads to an association between the receptor and a catalytic subunit. Such an association might make it more difficult to remove the enzyme from the surrounding lipid matrix of the membrane.