Freshly isolated epithelial cells from hen colon were investigated using the patch-clamp technique. The aim of this investigation was to characterise the cellular conducting site for Cl- secretion. In cell-attached mode two types of Cl(-)-channels were found. Both showed distinct outward rectification. The channel types differed in single channel conductances and the marked voltage dependence of the open probabilities. A low conductance Cl(-)-channel was observed with a mean conductance at negative holding potentials of g- = 9 pS, and of g+ = 34 pS at positive potentials. This channel was predominantly open at negative potentials, corresponding to cell hyperpolarization. The second channel type observed had conductances of g- = 35 pS and g+ = 77 pS, and showed increasing open probabilities with increasing holding potentials (cell depolarisation). Both channel types were blockable by the Cl(-)-channel blocker NPPB. These data in combination with previously published transepithelial transport data on hen colon indicate that these channels are the Cl- secretory sites in colon epithelium.