Inappropriate activation of the renin-angiotensin system, which plays a central role in the regulation of blood pressure, electrolyte, and volume homeostasis, may represent a major risk factor for hypertension, heart attack, and stroke. Mounting evidence from clinical studies has demonstrated an inverse relationship between circulating vitamin D levels and the blood pressure and/or plasma renin activity, but the mechanism is not understood. We show here that renin expression and plasma angiotensin II production were increased severalfold in vitamin D receptor-null (VDR-null) mice, leading to hypertension, cardiac hypertrophy, and increased water intake. However, the salt- and volume-sensing mechanisms that control renin synthesis are still intact in the mutant mice. In wild-type mice, inhibition of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] synthesis also led to an increase in renin expression, whereas 1,25(OH)2D3 injection led to renin suppression. We found that vitamin D regulation of renin expression was independent of calcium metabolism and that 1,25(OH)2D3 markedly suppressed renin transcription by a VDR-mediated mechanism in cell cultures. Hence, 1,25(OH)2D3 is a novel negative endocrine regulator of the renin-angiotensin system. Its apparent critical role in electrolytes, volume, and blood pressure homeostasis suggests that vitamin D analogues could help prevent or ameliorate hypertension.