These experiments were designed to test the hypothesis that renal renin is an important determinant of the severity of acute renal failure in rats. Two-kidney "Goldblatt rats" were prepared by constricting the left renal arteries with silver clips and leaving the contralateral arteries untouched. After 2 to 5 weeks, the clips were removed, and HgCl2 was injected in 13 rats (1 ml/kg body weight of 4.7 mg of HgCl2 per milliliter of 140 mM NaCl). These rats exhibited the characteristic features of acute renal failure 24 hr later. As compared with a group of seven similarly treated rats injected with 140 mM NaCl without HgCl2, GFR, V, and UNaV were reduced, and %FE H2O and %FE Na were increased. As assessed by these parameters, severity of functional impairment was equal in both kidneys. However, cortical renin was 28 times higher in the left kidney than in the right. These results are inconsistent with the hypothesis.