A feature common to all adenylate cyclase isoforms is the ability to integrate multiple stimulatory and inhibitory signals leading to the production of cyclic AMP levels corresponding to the stimuli. The least characterized of the nine adenylate cyclase isoforms is AC9. AC9 is a unique member of the mammalian adenylate cyclase family. It is the most divergent in sequence and unlike the other isoforms, AC9 is relatively insensitive to activation by forskolin. AC9 is expressed widely in the central nervous system as well as in other major organs. Of particular interest is the relative abundance of AC9 message and protein in the hippocampus, an important region for certain forms of learning and memory. Understanding the regulatory features of AC9 will be key to interpreting and manipulating cyclic AMP responses in these tissues.