In this study we have examined the effects of prostaglandin E 2 (PGE 2 ), the cyclooxygenase inhibitor, indomethacin, and a protein kinase A inhibitor (PKA-I) on the Cl - conductance in isolated zymogen granules (ZG) from cholecystokinin octapeptide (CCK-8) pre-stimulated pancreatic acini. The Cl - conductance in isolated ZG from CCK-8 pre-stimulated rat pancreatic acini increases with increasing CCK-8 concentrations and decreases at supramaximal CCK-8 concentrations. The basal and CCK-8-stimulated Cl - conductance in ZG is inhibited by pretreatment of acini with PGE 2 (10 - 6 M). This PGE 2 -induced inhibition is abolished in the presence of PKA-I (20 U/ml). Furthermore, pretreatment of acini with indomethacin (10 - 5 M) or PKA-I (20 U/ml) abolishes the decrease in the Cl - conductance at supramaximal CCK-8 concentrations (10 - 9 M). We conclude that the inhibition of the Cl - conductance in isolated ZG at high CCK-8 concentrations is mediated by an enhanced production of PGE 2 , and that PGE 2 operates by stimulating adenylate cyclase (AC) with a consequent rise in cAMP and activation of PKA.