The crystal structure of a soluble, catalytically active form of adenylyl cyclase in a complex with its stimulatory heterotrimeric G protein alpha subunit (G(S alpha)) and forskolin was determined to a resolution of 2.3 angstroms. When P-site inhibitors were soaked into native crystals of the complex, the active site of adenylyl cyclase was located and structural elements important for substrate recognition and catalysis were identified. On the basis of these and other structures, a molecular mechanism is proposed for the activation of adenylyl cyclase by G(S alpha).