Activation of K+/Cl- cotransport was studied after exposure of normal human erythrocytes to the oxidative action of acetylphenylhydrazine (APH), menadione sodium bisulfite (MSB), hydrogen peroxide (H2O2) or phenazine metasulfate (PMS). In order to better define the relative contributions of K+/Cl- cotransport on ouabain and bumetanide-resistant (OBR) K+ efflux induced by oxidation, we used (dihydroindenyl)oxyalkanoic acid (DIOA) and carbocyanine as specific inhibitors, respectively, of cotransport system and Ca(2+)-activated K+ channel. APH, MSB and - to much less extent - H2O2 promoted a K+ efflux pathway with features corresponding to those of K+/Cl- cotransport. This pathway showed: (i) kinetics of efflux compatible with a specific cation transport system; (ii) requirement for chloride anion; (iii) resistance to ouabain, bumetanide and carbocyanine inhibition; (iv) stimulation by hypotonic challenge; (v) susceptibility to inhibition by DIOA. Dithiothreitol (DTT) or 2-mercaptoethanol (2-ME) decreased K+/Cl- cotransport activation, suggesting that oxidative mechanisms affected crucial SH groups of the transporter. These data suggest that oxidation represents a factor capable of modulating activation of K+/Cl- cotransport. Its possible contribution in situations with high oxidative risk, such as sickle-cell anaemia or beta thalassemia, is discussed.