1. During osmotic swelling, cultured human small intestinal epithelial cells (Intestine 407) exhibited activation of large Cl- currents under the patch-clamp whole-cell configuration. The volume-sensitive Cl- conductance was independent of intracellular Ca2+ and cyclic AMP. 2. The anion permeability sequence of the current was SCN- > I- > Br- > Cl- > F- > gluconate-, corresponding to Eisenman's sequence I. 3. Cl- currents were instantaneously activated by command pulses in a range of -120 to +45 mV. At potentials more positive than +50 mV the current showed a time-dependent inactivation. This inactivation was accelerated by increased depolarization. The instantaneous current-voltage relationship rectified in the outward direction. 4. A stilbene-derivative Cl- channel blocker, 4-acetamido-4'-isothiocyanostilbene (SITS), inhibited the Cl- current at micromolar concentrations. SITS facilitated inactivation at positive potentials. Outward currents were more prominently suppressed by SITS than inward currents. The concentrations required for 50% inhibition (IC50) of outward and inward currents were 1.5 and 6 microM, respectively. The outward and inward currents were equally inhibited by a carboxylate analogue Cl- channel blocker, 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) or diphenylamine-2-carboxylate (DPC) at higher doses (IC50 = 25 for NPPB or 350 microM for DPC). Inactivation kinetics at large depolarizations was not affected by NPPB or DPC. 5. The Cl- current was blocked by an unsaturated fatty acid, arachidonic acid (IC50 = 8 microM). Arachidonic acid was still effective in the presence of inhibitors of lipoxygenase (nordihydroguaiaretic acid, 10 microM), cyclo-oxygenase (indomethacin, 10 microM) and protein kinase C (polymyxin B, 30 microM). The Cl- current was also sensitive to another cis unsaturated fatty acid, oleic acid, which is not a substrate for oxygenases. A trans isomer of oleate, elaidic acid, and a saturated fatty acid, palmitic acid, were ineffective. 6. Single Intestine 407 cells exposed to a hypotonic solution showed a regulatory volume decrease after initial osmotic swelling. The volume regulation was abolished by SITS, NPPB, arachidonate and oleate, but not by elaidate and palmitate. 7. It is concluded that outwardly rectifying Cl- channels, which are sensitive to arachidonic acid, are activated upon osmotic swelling and involved in the subsequent cell volume regulation.