The important cardiovascular regulator renin contains a strong in vitro enhancer 2.7 kb upstream of its gene. Here we tested the in vivo role of the mouse Ren-1 c enhancer. In reninexpressing As4.1 cells stably transfected with Ren-1 c promoter with or without enhancer, expression of linked -geo reporter, stable expression, and colony formation were dependent on the presence of the enhancer. We then generated mice carrying a targeted deletion of the enhancer (REKO mice) and found marked depletion of renin in renal juxtaglomerular and submandibular ductal cells, as well as hyperplasia of macula densa cells. Plasma creatinine was increased, but electrolytes were normal. Male REKO mice implanted with telemetry devices had 9 ؎ 1 mm Hg lower mean arterial pressure ( p < 0.001), which was partly normalized by a high NaCl diet. Locomotor activity was lower, and baroreflex sensitivity was normal. Markedly reduced mean arterial pressure variability in the midfrequency band indicated a contribution of reduced sympathetic vasomotor tone to the hypotension. In conclusion, the renin enhancer is critical for renin gene expression and physiological sequelae, including response to alteration in salt intake. The REKO mouse may be useful as a low renin expression model. Renin, produced in renal juxtaglomerular (JG) 4 cells, is the rate-limiting enzyme of the renin-angiotensin system that generates angiotensin II. The renin-angiotensin system helps maintain fluid and electrolyte balance and blood pressure (BP) but also has important roles in development, vascular hypertrophy, angiogenesis, and clinical conditions, such as renal hypertension and heart failure (1). The JG cells are modified myoepithelial cells in the wall of the afferent arteriole and are in close contact with the macula densa segment of the distal tubule that signals the renal arterioles to regulate glomerular filtration rate and renin secretion (2).Transcription of renin genes is regulated in both a tissuespecific and developmental manner (1, 3). In the mouse kidney, renin expression is first detected at embryonic day 14.5 in the developing arteries and renal arterial branches. Expression then becomes progressively restricted to smaller arteries and arterioles, eventually contracting to just the JG cells postpartum (4). In addition to the kidney, mice also express renin in the submandibular gland (SMG) and, at lower levels, in adrenal, brain, and various other tissues.To ascertain the importance of renin, strains of mice have been generated with deletion of the entire renin gene (termed Ren-1 c or Ren-1 d , depending on mouse strain). Ren-1 c null mice show neither detectable plasma renin activity nor plasma angiotensin I, a reduction in BP of 20 -30 mm Hg, increased urine output and water intake, and altered renal morphology (5), as also seen in angiotensinogen-deficient mice (6, 7). Consistent with this, Ren-1 d null mice show altered macula densa morphology, complete absence of JG cell granulation, and sexually dimorphic hypotension (8).An understanding of t...