Thiol reagents activate K-Cl cotransport (K-Cl COT), the Cl-dependent and Na-independent ouabain-resistant K flux, in red blood cells (RBCs) of several species, upon depletion of cellular glutathione (GSH). K-Cl COT is physiologically active in high potassium (HK), high GSH (HG) dog RBCs. In this unique model, we studied whether the same inverse relationship exists between GSH levels and K-Cl COT activity found in other species. The effects of GSH depletion by three different chemical reactions [nitrite (NO₂)-mediated oxidation, diazene dicarboxylic acid bis-N,N-dimethylamide (diamide)-induced dithiol formation, and glutathione S-transferase (GST)-catalyzed conjugation of GSH with 1-chloro-2,4-dinitrobenzene (CDNB)] were tested on K-Cl COT and regulatory volume decrease (RVD). After 85% GSH depletion, all three interventions stimulated K-Cl COT half-maximally with the following order of potency: diamide > NO₂ > CDNB. Repletion of GSH reversed K-Cl COT stimulation by 50%. Cl-dependent RVD accompanied K-Cl COT activation by NO₂ and diamide. K-Cl COT activation at concentration ratios of oxidant/GSH greater than unity was irreversible, suggesting either nitrosothiolation, heterodithiol formation, or GST-mediated dinitrophenylation of protein thiols. The data support the hypothesis that an intact redox system, rather than the absolute GSH levels, protects K-Cl COT activity and cell volume regulation from thiol modification.