Excised roots of barley (Hordeum vulgare, var. Campana) were incubated in KCl, K(2)SO(4), CaCl(2), and NaCl solutions at concentrations of 10(-5) to 10(-2)n. Changes in substrate solution pH, cell sap pH, and organic acid content of the roots were related to differences in cation and anion absorption. The pH of expressed sap of roots increased when cations were absorbed in excess of anions and decreased when anions were absorbed in excess of cations. The pH of the cell sap shifted in response to imbalances in cation and anion uptake in salt solutions as dilute as 10(-5)n. Changes in cell sap pH were detectable within 15 minutes after the roots were placed in 10(-3)n K(2)SO(4). Organic acid changes in the roots were proportional to expressed sap pH changes induced by unbalanced ion uptake. Changes in organic acid content in response to differential cation and anion uptake appear to be associated with the low-salt component of ion uptake.