The activation energy of adenylate cyclase by p[NH]ppG in rat pancreatic plasma membranes was estimated to be 141-189 kj/mol. When a high concentration of secretin or CCK-8 (C-terminal octopeptide of cholecystokinin-pancreozimin) was added to the assay medium, the activation energy was reduced to 73 kj/mol. This hormone effect was exerted on the activation energy of the activation process of adenylate cyclase by p[NH]ppG. Indeed, when plasma membranes were preactivated with p[NH]ppG alone or with p[NH]ppG and CCK-8 and then washed, there resulted a persistent activation with low activation energy (65 and 48 kj/mol, respectively). A similar low activation energy was observed in membranes preincubated with GMP and CCK-8. The latter treatment could not induce persistent activation but facilitated the activation by p[NH]ppG, suggesting that the step of p[NP]ppG activation requiring a high activation energy in the absence of hormone had developed during preincubation with GMP and CCK-8, and had not been reversed by membrane washing. By contrast, EDTA pretreatment did not influence p[NH]ppG activation while provoking a reversible deactivation of persistently activated adenylate cyclase.