Mice lacking Ren1c were generated using C57BL/6-derived embryonic stem cells. Mice homozygous for Ren1c disruption (Ren1c-/-) are born at the expected ratio, but approximately 80% die of dehydration within a few days. The surviving Ren1c-/- mice have no renin mRNA expression in the kidney, hydronephrosis, thickening of renal arterial walls, and fibrosis in the kidney. Plasma renin and angiotensins I and II are undetectable. Urinary aldosterone is 6% wild-type. They have low tail-cuff BP (84 +/- 4 versus 116 +/- 5 mmHg in +/+) and excrete large amounts of urine (5.2 +/- 0.8 ml/d, 725 +/- 34 mOsm versus 1.1 +/- 0.1 ml/d, 2460 +/- 170 mOsm in +/+). After 5 d of drinking 5% dextrose, desmopressin does not increase the osmolality of the urine in -/- mice (624 +/- 19 to 656 +/- 25 mOsm), whereas in +/+, it increases severalfold (583 +/- 44 to 2630 +/- 174 mOsm). Minipump infusion of angiotensin II to Ren1c-/- mice restores BP to wild-type level, but preexisting damage to the medulla prevents complete restoration of the ability of the kidney to concentrate urine. Heterozygous Ren1c+/- mice, in contrast, are indistinguishable from +/+ in BP, urine volume, and osmolality. Kidney renin mRNA, the number of kidney cells producing renin, and plasma renin concentration in the Ren1c+/- mice are also indistinguishable from +/+. These results demonstrate that renin is the only enzyme capable of maintaining plasma angiotensins and that renin expression in the kidney is very tightly regulated at the mRNA level.