Three different splice-isoforms of the Na/K/2Cl cotransporter, NKCC2, are expressed along the thick ascending limb of Henle: NKCC2A, NKCC2B, and NKCC2F. The reason for the existence of 3 different NKCC2 isoforms is unclear. To address the role of NKCC2A in kidney function we generated NKCC2A-/- mice. Ambient urine osmolarity was lower in NKCC2A-/- mice (1384±98 mosm/l; n=15) compared to WT (1910±173 mosm/l; n=14; p=.012), but urinary concentrating ability after water restriction was preserved. Baseline plasma renin concentration (PRC) was similar in NKCC2A-/- (640±85 ng Ang I/ml/hr; n=20) and +/+ mice (629±79 ng Ang I/ml/hr; n=19; p=.92). However, the suppression of PRC after acute salt loading (saline, i.v.), as a measure of macula densa-mediated suppression of renin secretion was only -16±2% in NKCC2A-/- mice (n=17; p=.14 vs. basal) compared to -42±6% in WT (n=13; p=.006 vs. basal). Chronic high salt intake, conversely, suppressed PRC similarly in both genotypes. Maximum tubuloglomerular feedback responses as determined by stop flow pressure (PSF) measurements were significantly reduced in NKCC2A-/- (PSF=-16±2%) compared to WT (PSF=-26±3%; p=.016). In summary, NKCC2A-deficient mice are viable and show no severe salt-losing phenotype. However, NKCC2A activity seems to be crucially involved in macula densa salt sensing in the high Cl concentration range.