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The electroneutral Na + -dependent HCO 3 − transporter NBCn1 is strongly expressed in the basolateral membrane of rat medullary thick ascending limb cells (mTAL) and is up-regulated during NH 4 + -induced metabolic acidosis. Here we used in vitro perfusion and BCECF video-imaging of mTAL tubules to investigate functional localization and regulation of Na + -dependent HCO 3 − influx during NH 4 + -induced metabolic acidosis. Tubule acidification was induced by removing luminal Na + (∆pH i : 0.88 ± 0.11 pH units, n = 10). Subsequently the basolateral perfusion solution was changed to CO 2 /HCO 3 − buffer with and without Na + . Basolateral Na + -H + exchange function was inhibited with amiloride. Na + -dependent HCO 3 − influx was determined by calculating initial base flux of Na + -mediated re-alkalinization. In untreated animals base flux was 8.4 ± 0.9 pmol min −1 mm −1 . A 2.4-fold increase of base flux to 21.8 ± 3.2 pmol min −1 mm −1 was measured in NH 4 + -treated animals (11 days, n = 11). Na + -dependent re-alkalinization was significantly larger when compared to control animals (0.38 ± 0.03 versus 0.22 ± 0.02 pH units, n = 10). In addition, Na + -dependent HCO 3 − influx was of similar magnitude in chloride-free medium and also up-regulated after NH 4 + loading. Na + -dependent HCO 3 − influx was not inhibited by 400 µm DIDS. A strong up-regulation of NBCn1 staining was confirmed in immunolabelling experiments. RT-PCR analysis revealed no evidence for the Na + -dependent HCO 3 − transporter NBC4 or the two Na + -dependent CI − /HCO 3 − exchangers NCBE and NDCBE. These data strongly indicate that rat mTAL tubules functionally express basolateral DIDS-insensitive NBCn1. Function and protein are strongly up-regulated during NH 4 + -induced metabolic acidosis. We suggest that NBCn1-mediated basolateral HCO 3 − influx is important for basolateral NH 3 exit and thus NH 4 + excretion by means of setting pH i to a more alkaline value.