Fibrates are peroxisome proliferator-activated receptor-α (PPARα) ligands in widespread clinical use to lower plasma triglyceride levels. We investigated the effect of fenofibrate and clofibrate on ion transport in mouse intestine and in human T84 colonic adenocarcinoma cells through the use of short-circuit current ( Isc) and ion flux analysis. In mice, oral administration of fenofibrate produced a persistent inhibition of cAMP-stimulated electrogenic Cl−secretion by isolated jejunum and colon without affecting electroneutral fluxes of22Na+or86Rb+(K+) across unstimulated colonic mucosa. When applied acutely to isolated mouse intestinal mucosa, 100 μM fenofibrate inhibited cAMP-stimulated Iscwithin 5 min. In T84 cells, fenofibrate rapidly inhibited ∼80% the Cl−secretory responses to forskolin (cAMP) and to heat stable enterotoxin STa (cGMP) without affecting the response to carbachol (Ca2+). Both fenofibrate and clofibrate inhibited cAMP-stimulated Iscwith an IC50∼1 μM, whereas other PPARα activators (gemfibrozil and Wy-14,643) were without effect. Membrane permeabilization experiments on T84 cells indicated that fenofibrate inhibits basolateral cAMP-stimulated K+channels (putatively KCNQ1/KCNE3) without affecting Ca2+-stimulated K+channel activity, whereas clofibrate inhibits both K+pathways. Fenofibrate had no effect on apical cAMP-stimulated Cl−channel activity. Patch-clamp analysis of HEK-293T cells confirmed that 100 μM fenofibrate rapidly inhibits K+currents associated with ectopic expression of human KCNQ1 with or without the KCNE3 β-subunit. We conclude that fenofibrate inhibits intestinal cAMP-stimulated Cl−secretion through a nongenomic mechanism that involves a selective inhibition of basolateral KCNQ1/KCNE3 channel complexes. Our findings raise the prospect of fenofibrate as a safe and effective antidiarrheal agent.