Addition of H2O2 (0.5 mM) to Ehrlich ascites tumor cells under isotonic conditions results within 25 min in a substantial (22 +/- 1 %) reduction in cell volume. The cell shrinkage is paralleled by net loss of K(+), which was significant within 8 min, whereas no concomitant increase in the K(+) or Cl(-) conductances could be observed. The H2O2-induced cell shrinkage was unaffected by the presence of clofilium and clotrimazole, that block volume-sensitive and Ca(2+)-activated K(+) channels, respectively, and unaffected by a raise in extracellular K(+) concentration to a value which eliminates the electrochemical driving force for K(+). On the other hand, the H2O2-induced cell shrinkage was impaired in the presence of the KCl cotransport inhibitor DIOA, following substitution of NO3(-) for Cl(-), and when the driving force for KCl cotransport was omitted. It is suggested that H2O2 activates electro neutral KCl cotransport in Ehrlich ascites tumor cells and not K(+) and Cl(-) channels. Addition of H2O2 to hypotonically exposed cells accelerates the regulatory volume decrease and the concomitant net loss of K(+), whereas no additional increase in the K(+) and Cl(-) conductance was observed. The effect of H2O2 on cell volume was blocked by the serine/threonine phosphatase inhibitor calyculin A, indicating an important role of serine/threonine phosphorylation in the H2O2 mediated activation of KCl cotransport in Ehrlich cells. In contrast, addition of H2O2 to adherent cells, e.g., Ehrlich Lettre ascites cells, a subtype of the Ehrlich ascites tumor cells, and NIH3T3 mouse fibroblasts increased the K(+) and Cl(-) conductances after hypotonic cell swelling. Hence, H2O2 induces KCl cotransport or K(+) and Cl(-) channels in non-adherent and adherent cells, respectively. Key words: Reactive oxygen species, Cell volume Regulation, Cotransporter, Taurine.