Endothelin, a novel potent vasoconstrictor peptide produced by vascular endothelial cells, stimulated anion secretion by a cultured secretory epithelium derived from the rat epididymis as measured by changes in short-circuit current (SCC). Stimulation of the SCC was observed when endothelin was added to the basolateral or the apical side of the epithelium. The response to basolateral application was greater than that to apical application. The EC50 values were found to be 1.3 and 3.0 nM for basolateral and apical application, respectively. These values were about one-half to one order of magnitude higher than that required for its vasoconstrictor action. The stimulation of SCC by endothelin was likely to be due to an increase to anion secretion as removal of Cl from the incubation medium markedly reduced the SCC response to endothelin. Diphenylamine-2-carboxylate (DPC, 0.1 mM), a Cl-channel blocker, added to the apical side also inhibited the endothelin-induced rise in SCC. The stimulation of SCC by endothelin was accompanied by a rise in the intracellular cyclic AMP content in epididymal monolayers. Immunofluorescence staining has shown the presence of immunoreactive endothelin-like compound in the interstitium and epithelial cells of the rat epididymis. It is speculated that endogenous endothelin plays an important role in the control of water and electrolyte transport in the epididymis.