In a previous proteomic study, we found dramatic differences in fumarase in the kidney between Dahl salt-sensitive rats and salt-insensitive consomic SS-13 rats. Fumarase catalyzes the conversion between fumarate and l-malate in the tricarboxylic acid cycle. Little is known about the pathophysiological significance of fumarate metabolism in cardiovascular and renal functions, including salt-induced hypertension. The fumarase gene is located on the chromosome substituted in the SS-13 rat. Sequencing of fumarase cDNA indicated the presence of lysine at amino acid position 481 in Dahl salt-sensitive rats and glutamic acid in Brown Norway and SS-13 rats. Total fumarase activity was significantly lower in the kidneys of Dahl salt-sensitive rats compared with SS-13 rats, despite an apparent compensatory increase in fumarase abundance in Dahl salt-sensitive rats. Intravenous infusion of a fumarate precursor in SS-13 rats resulted in a fumarate excess in the renal medulla comparable to that seen in Dahl salt-sensitive rats. The infusion significantly exacerbated salt-induced hypertension in SS-13 rats (140±3 vs125±2 mm Hg in vehicle control at day 5 on a 4% NaCl diet; P<0.05). In addition, the fumarate infusion increased renal medullary tissue levels of H2O2. Treatment of cultured human renal epithelial cells with the fumarate precursor also increased cellular levels of H2O2. These data suggest a novel role for fumarate metabolism in salt-induced hypertension and renal medullary oxidative stress. © 2009 American Heart Association, Inc.