To address the in vivo relevance of the Macula densa (MD) control of renin secretion, mice were generated lacking specifically the B-isoform of the Na/K/2Cl cotransporter NKCC2, believed to be expressed in the MD. NKCC2B-/- mice showed suppressed basal plasma renin concentrations (PRC) averaging 1089±81 (n=35) compared to 1448±85 (n=28) ng Ang I/ml/h in wild types (p=.0035). After a high salt diet (4% NaCl, one week), PRC was suppressed by 38% in the WT and by 59% in the KO mice to 901±108 (n=9; p=.003 vs. normal diet) and 445±71 ng Ang I/ml/h (n=10; p=.0003 vs. normal diet; p=.0023 between genotypes). Conversely, a low salt diet (0.01% NaCl) stimulated PRC to the same extent averaging 2447±310 (n=10; p=.0002 vs. control diet) and 2615±413 ng Ang I/ml/h (n=9; p<.0001 vs. control diet) in WT and KO mice, respectively (p=.75). NKCC2B deficiency was accompanied by increased distal Cl concentrations (micropuncture): at perfusion rates of 6 and 15 nl/min distal Cl concentrations were 70.2±3.9 vs. 45.3±3.6 and 93.1±4.5 vs. 74.0±5.2 mmol/l Cl; p<.0001 and p=.009 vs. WT, respectively. ISH revealed co-expression of the NKCC2A isoform in the MD. We conclude that the increased chloride concentration at the MD in NKCC2B-/- mice is sensed by the remaining NKCC2A and leads to a suppression of renin secretion, a direct in vivo evidence for the macula densa control of renin secretion.