Combined intracellular and transepithelial potential and resistance measurements were performed to localize the ion conductances activated by hypo-osmotic shock of cultured human colonic carcinoma cells (HT-29Cl.19A). Furthermore, the effect of cell swelling induced by a hypo-osmotic solution on the intracellular Ca2+ activity [Ca2+]i and release of amino acids into the extracellular solution was examined. Application of a 40% hypo-osmotic solution on both sides of confluent monolayers induced a hyperpolarization of the intracellular potential caused by increased K+ conductance of the basolateral membrane, followed by a sustained depolarization due to increased Cl- conductance in the apical and basolateral membranes. Usually no transepithelial current occurred, presumably because of random distribution of Cl- channels. However, in some monolayers cell swelling induced a transepithelial Cl- current because of a more pronounced expression of volume-sensitive Cl- channels in the apical membrane. Exposure to hypo-osmotic solution increased [Ca2+]i transiently. The increase of [Ca2+]i was also observed to occur in the presence of the muscarinic receptor agonist carbachol or the inhibitor of the microsomal Ca2+-ATPase thapsigargin (TG), which prevented carbachol-induced Ca2+ release, suggesting that cell swelling recruits Ca2+ from a different source compared to carbachol or TG. Following incubations with hypo-osmotic solutions, about 60% of the intracellular free amino acids including aspartate, glutamate, glycine and taurine was released. It is concluded that the regulatory volume decrease (RVD) in HT-29Cl.19A colonocytes is achieved by activation of K+ and Cl- conductances, resulting in net loss of salt, as well by extrusion of amino acids.