Objective: Cell volume changes are of critical importance in many physiological processes, such as salt and water transport, migration, apoptosis. The response of cells to such challenges involves a regulatory volume decrease (RVD), during which K⁺ channels are activated as a general rule. It is the aim of this study to analyze the effect of cell volume changes on the Na⁺ and Cl^- activated, high-conductance Slick K⁺ channels (Slo2.1) and explore the possible mechanisms for this type of regulation. Methods: We have co-expressed Slick K⁺ channels with aquaporins in Xenopus laevis oocytes and subsequently induced changes in cell volume, of approx. 5%, by exposure to hypo- or hypertonic media. Currents were measured by two- electrode voltage clamp. Results: The high- conductance Slick channel (Slo2.1) is dramatically stimulated (to 195 % of control) by cell swelling and inhibited (to 52 % of control) by a decrease in cell volume. Surprisingly, treatment of the oocytes with cytochalasin D suggested that the regulation of Slick channels does not depend on an intact actin cytoskeleton. Application of Apyrase, an ATP diphosphohydrolase, indicated that ATP release during cell swelling does not mediate the regulation of the Slick K⁺ channels. Finally, incubation with Brefeldin A showed that increased currents observed under hypotonic solutions are not dependent on the exocytic pathway. Conclusion: Slick K⁺ channels are dramatically regulated by small, fast changes in cell volume. This is the only high-conductance K⁺ channel, which is highly sensitive to small changes in cell volume. The regulation is not mediated by the cytoskeleton, ATP release or delivery of newly synthesized protein to the cell membrane. Thus the mechanism for regulation is still obscure and needs further investigation.