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Physiological roles of the renin-angiotensin system in maintaining blood pressure and sodium-water balance in angiotensinogen gene-knockout mice were evaluated with special reference to endogenous pressor substances.Angiotensinogen-gene knockout mice and control mice were fed a 0.3 or 4% NaCl diet for 2 weeks. Systolic blood pressure and urinary excretions of electrolytes, creatinine, aldosterone, adrenaline, noradrenaline, dopamine and vasopressin were measured.About 60% of our angiotensinogen-gene knockout mice did not survive until weaning. These mice presented with hypotension and polyuria. Urinary excretion of aldosterone from such mice was significantly lower (not detected) than that from control mice (2.0 ± 0.3 pg/mg creatinine). In contrast, urinary excretion of vasopressin from angiotensinogen-gene knockout mice (0.7 ± 0.1 ng/mg creatinine) was greater than that from control mice (0.3 ± 0.1 ng/mg creatinine), and those of adrenaline and of noradrenaline were similar for knockout and control mice. After salt loading (a 4% NaCl diet), angiotensinogen-gene knockout mice exhibited a significant increase in systolic blood pressure (from 68.3 ± 2.9 to 95.9 ± 5.9 mmHg), significant decreases in urinary excretions of adrenaline (from 65 ± 8 to 40 ± 7 pg/mg creatinine) and noradrenaline (from 467 ± 48 to 281 ± 41 pg/mg creatinine) and no change in excretion of vasopressin compared with such mice fed a 0.3% NaCl diet.The present results with angiotensinogen gene knockout mice confirm that the renin-angiotensin system plays fundamental roles in maintaining the blood pressure and sodium-water balance. Because the vasopressin and catecholaminergic systems may be altered by lack of angiotensin in angiotensinogen-gene knockout mice, these systems perhaps are not able to restore blood pressure and sodium-water depletion to normal levels in these mice. J Hypertens 16:285–289 © 1998 Rapid Science Ltd.