.Changes in activity or expression of transporters may account for alterations in cell behavior in diabetes. We sought to ascertain if mesangial cells (MC) grown in different glucose concentrations exhibit changes in activity and expression of acid-extruding transporters, the Na⁺/H⁺ and Na⁺-dependent Cl/HCO₃ exchanger. pH_i was determined by the use of the fluorescent pH-sensitive dye BCECF. In MCs grown in 5 mM glucose (control), the Na⁺/H⁺ exchanger was responsible for 31.8 ± 5.1% of steady-state pH_i, whereas Na⁺-dependent Cl/HCO₃ contributed 62.9 ± 4.0% (n = 11). In MCs grown in high glucose for 2 wk, Na⁺/H⁺ exchange contribution to acid-extrusion increased as follows: 42.3 ± 4.6% [n = 8, 10 mM, not significant (NS)], 51.1 ± 5.1% (n = 8, 20 mM, P < 0.01), and 64.8 ± 5.5% (n = 7, 30 mM, P < 0.001). The Na⁺-dependent Cl/HCO₃ exchanger contributed less [47.0 ± 4.6, 38.6 ± 5.8, and 21.1 ± 3.8%, for 10, 20, and 30 mM glucose, respectively (n > 7)]. We sought to ascertain if the magnitude of the acute stimulated response to ANG II by the Na⁺/H⁺ and Na⁺-dependent Cl/HCO₃ exchanger is changed. Na⁺/H⁺ exchanger (1.89-fold increase in 30 vs. 5 mM, P < 0.002), but not Na⁺-dependent Cl/HCO₃ exchange (0.17-fold, NS), exhibited an enhanced response to ANG II (1 µM). Na⁺/H⁺ exchange (NHE1) expression was significantly different (1.72-fold) after prolonged exposure to high glucose. These results suggest that the Na⁺/H⁺ exchanger, but not Na⁺-dependent Cl/HCO₃ exchanger, may play an early role in the response to hyperglycemia in the diabetic state.