The mature rat testis contains two forms of adenylate cyclase. One is membrane-bound and hormone-responsive; the other is water-soluble and not activated by hormones (Braun, T., and Dods, R. F. (1976) Pnoc. Natl. Acad. Sci. U.S.A. 72, 1097-1101). The water-soluble adenylate cyclase is a globular protein which is much smaller than the enzyme which is solubilized from membranes of mature rat testis by Triton X-100. Its physical properties are: sedimentation coefficient, 3.8 S; Stokes radius, 34 A; molecular weight, 56,000; frictional ratio, 1.2. Inclusion of protease inhibitors during enzyme extraction did not affect the fraction of total enzyme activity which was water-soluble, nor did autolysis for 1 h at 25 degrees C. The physical properties of the membrane-bound adenylate cyclase were determined after solubilization with Triton X-100. In detergent, the values are: sedimentation coefficient, 6.7 S; Stokes radius, 68 A; partial specific volume, 0.73 ml/g; molecular weight, 191,000; frictional ratio, 1.6. The fact that the partial specific volume of the enzyme in detergent is the same as that of a typical water-soluble protein shows that the Triton X-100-solubilized enzyme does not bind a large quantity of detergent. This indicates that it does not have extensive hydrophobic regions on its surface. The detergent-solubilized adenylate cyclase of the mature rat testis is similar in this respect to adenylate cyclase solubilized from the rat renal medulla (Neer, E. J. (1974) J. Biol. Chem. 249, 6527-6531).