Forskolin, a naturally occurring diterpene, directly stimulates adenylyl cyclase and has been used extensively to increase cAMP and to elicit cAMP-dependent physiological responses. More recently, forskolin has been shown to inhibit a number of membrane transport proteins and channel proteins through a mechanism that does not involve the production of cAMP. Many of these channel proteins are predicted to have similar topographies in the membrane bilayer and it is tempting to speculate that forskolin may be binding at structurally homologous sites. Kenneth Seamon and colleagues discuss the cAMP-independent effects of forskolin and the structural similarity between forskolin and other physiologically important substances such as hexoses and steroids with respect to potential forskolin binding sites.