Na/HCO3 cotransporters (NBCs) are important regulators of intracellular pH (pH i ) in a variety of organ systems where acid-base status is critical for tissue function. To characterize the pharmacology of NBCs in more detail, we used the two-electrode voltage-clamp technique to examine the effect of previously identified inhibitors of anion exchanger 1 (AE1) on the activity of rat NBCe1-A expressed in Xenopus laevis oocytes. NBC-expressing oocytes voltage-clamped at -60 mV and exposed to a 5% CO2/33 mM HCO 3- solution displayed NBC-mediated outward currents that were inhibited by either niflumic acid or one of the two bis-oxonol dyes diBA(3)C4 and diBA(5)C4. NBCe1-A was less sensitive to niflumic acid (apparent K i of 100 μM) than 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid (DIDS, apparent K i of 36 μM) but more sensitive to the diBAC dyes (apparent K i of ∼10 μM). Based on current-voltage relationships, the diBAC dyes inhibited HCO 3- -induced NBCe1-mediated inward currents more so than outward currents. NBCe1 sensitivity to the dyes was (1) lower in the presence of 40 μM DIDS, (2) unaffected by changes in external HCO 3- concentration and (3) only modestly higher at an external Na+ concentration of 5, but not 15 or 33, mM. Therefore, the diBAC dyes compete with DIDS but not appreciably with Na + or HCO 3- for binding. The mechanism of diBAC inhibition of NBCe1 appears similar to that previously reported for AE1. © 2007 Springer Science+Business Media, LLC.