The catalytic subunit of rat brain adenylate cyclase was separated from its stimulatory subunit with loss of activation by 5'-guanylylimidodiphosphate. With respect to other properties, the resolved catalytic subunit was similar to the native enzyme. Nucleotide activation was restored after reconstitution with the isolated regulatory subunit. A key step in resolution was preliminary exposure of isolated membranes to increased ionic strength. After solubilization with nonionic detergent solution, the subunits were separated by gel filtration. The apparent molecular size of the catalytic subunit was unchanged by salt treatment, whereas the Stokes' radius of the regulatory subunit decreased from 72 A to 48 A.