The effect of chloride, nitrate and sulfate on stomatal opening in epidermal strips of C. benghalensis was examined in relation to phosphoenolpyruvate (PEP) carboxylation and malate production. Inclusion of malate, oxalacetate or bisulfite in the medium inhibited PEP carboxylase activity in epidermal tissues, suppressed malate formation and restricted stomatal opening. Malate ions therefore seem to favour potassium uptake even in presence of chloride. Stomata could open when supplied with K⁺ (K-Tricine); additional presence of chloride or nitrate enhanced further K⁺ - stimulated stomatal opening but reduced malate production and potassium content of epidermal tissues. Potassium content of epidermal tissues was higher when no inorganic anion was supplied than that in the presence of chloride or nitrate. The relative stimulation of stomatal opening by chloride or nitrate was greater in the presence of 10 m-equiv. K⁺ than at 100 m-equiv. K⁺. It is suggested that chloride as well as nitrate is taken up by guard cells: (i) to contribute with K⁺ to the turgor of guard cells and (ii) to act as a counterion for malate ions.