The aim of the present study was to investigate whether bicarbonate buffer (CO2 + HCO3-) is required to sustain maximal NaCl transport in the cortical thick ascending limb of Henle's loop (cTAL) of the mouse. Transepithelial Na+ and Cl- net fluxes (JNa, JCl, pmol min-1 mm-1), measured by electron microprobe analysis, were similar irrespective of the presence or absence of CO2 + HCO3- in luminal and bathing solutions (JNaCl with CO2 + HCO3- = 203 +/- 25 pmol min-1 mm-1; JNaCl without CO2 + HCO3- = 213 +/- 13 pmol min-1 mm-1, n = 14). Furthermore the transepithelial potential difference, Vte, the transepithelial resistance, Rte, and the basolateral membrane potential, Vbl, were unaffected by CO2 + HCO3-. In the absence of CO2 + HCO3-, Vte was +17.0 +/- 1.7 mV (n = 9) (lumen positive), Rte was 28 +/- 2 omega cm2 (n = 9) and Vbl was -76 +/- 4 mV (n = 6). In the presence of CO2 + HCO3-, Vte, Rte and Vbl were +15.9 +/- 1.5 mV, 29 +/- 1 omega cm2 and -73 +/- 5 mV, respectively. 4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid (SITS; 0.1 mmol l-1) and amiloride (1 mmol l-1) added to the (CO2 + HCO3-)-containing lumen perfusate were without effect on Vte and Rte. Finally, the effect of furosemide (0.1 mmol l-1) on Vte and Vbl in the presence of CO2 + HCO3- was investigated. Furosemide reversibly decreased Vte from +13.7 +/- 1.1 mV to +1.7 +/- 0.7 mV (n = 6) and hyperpolarized Vbl from -70 +/- 1 to -89 +/- 3 mV (n = 5), suggesting passive distribution of Cl- across the basolateral membrane. In conclusion, these data suggest that active NaCl transport in the cTAL of the mouse does not require the presence of CO2 + HCO3-.