The role of the adenylate cyclase (Ac)-cAMP system in mediating deoxycholic acid (DOC)-induced fluid secretion was studied in the rat jejunum and colon in vivo using the AC inhibitor, RMI 12 330 A. A potent inhibitory effect of RMI 12 330 A on fluid secretion induced by cholera toxin was demonstrated in ligated rat jejunal loops. On the contrary, the changes of fluid movement in jejunal and colonic loops caused by DOC could not be influenced by RMI 12 330 A, and mucosal cAMP levels of colonic loops were not increased. Colonic mucosal permeability estimated by the 14C-erythritol clearance increased significantly during a 45-min exposure to 3 mmol DOC, and was not affected by RMI 12 330 A. These results do not support the theory that the AC-cAMP system plays an important role in DOC-induced intestinal fluid secretion and suggest that an increase in mucosal permeability is the predominant factor responsible for the secretagogue effect.