Cyclic AMP plays an important regulatory role in transport activity and proliferation of renal MDCK cells. This observation and the ability of MDCK cells to synthesize prostaglandins provided the impetus to assess the hypothesis that endogenous prostaglandins modulate cyclic AMP concentrations in MDCK cells. Three dissimilar cyclo-oxygenase inhibitors: acetylsalicylate, 5 , 8, 11, 14-eicosatetraynoate, and meclofenamate significantly decreased cellular cyclic AMP levels and inhibited basal prostaglandin E2 synthesis. On the other hand, three dissimilar stimulators of prostaglandin synthesis: bradykinin, Ca'+-ionophore A23187 and arachidonate, increased cellular cyclic AMP levels, and stimulated prostaglandin E2 synthesis. Acetylsalicylate inhibited the bradykinin-and A23187-evoked increases of cyclic AMP as well as that of prostaglandin E, synthesis. Prostaglandin E2, the major prostaglandin synthesized by MDCK cells, dose-dependently increased cAMP levels when added exogenously. Acetylsalicylate did not significantly affect increases of cyclic AMP evoked by exogenous prostaglandin E2, documenting that acetylsalicylate inhibited cellular cyclic AMP levels by decreasing endogenous prostaglandin synthesis, rather than by a direct effect on cyclic AMP metabolism. Other prostaglandins synthesized by MDCK cells, i.e., prostaglandins 12, 6- keto-Fl,, and Fzm added exogenously did not significantly affect MDCK cyclic AMP levels, suggesting that they were probably ineffective as endogenous modulators of cyclic AMP. Moreover, endogenous prostaglandin E, appeared four-to eightfold more potent as a stimulator of cyclic AMP levels than exogenous prostaglandin E, . The results support t h e concept that prostaglandin E2 is an endogenous cellular mediator that acts between an extracellular effector such as bradykinin and a second endogenous mediator of hormone action: cyclic AMP. The MDCK cell line, derived from dog kidney, has retained many of the physiological and biochemical characteristics of renal distal tubular epithelia including morphological polarity and the presence of brushborder (Rindler et al., 19791, the ability to transport salt and water (Misfeldt et al., 1976), to form high-resistance Louis, MO): Acetylsalicylic acid (ASA), arachidonic acid,