Forskolin is a positive inotropic-acting and blood pressure lowering agent which was isolated from the Indian plant Coleus forskohlii. In isolated heart tissue, forskolin activates a membrane bound adenylatecyclase and a cytoplasmic cAMP-dependent protein kinase to a much higher degree than does isoprenaline. This activation does not require the hormone receptor. In isolated and electrically stimulated left guinea pig atria, the adenylate-cyclase activation by forskolin is the prerequisite for the positive inotropic effect. We therefore postulate the adenylatecyclase activation to be correlated with the positive inotropic effect via an enhanced calcium uptake by the heart muscle cell.