Indices of renal injury and oxidative stress were examined in mice with deficiency of cytosolic Cu(2+)/Zn(2+) superoxide dismutase (SOD1-/-, KO) and their wild-type (WT) littermates with streptozotocin-induced diabetes. After 5 weeks of diabetes, KO diabetic (D) but not WT-D mice developed marked albuminuria, increases in glomerular content of transforming growth factor beta, collagen alpha1(IV), and nitrotyrosine, and higher glomerular superoxide compared with corresponding values in nondiabetics. After 5 months of diabetes, increases in these parameters, mesangial matrix expansion, renal cortical malondialdehyde content, and severity of tubulointerstitial injury were all significantly greater, whereas cortical glutathione was lower, in KO-D than in WT-D. In contrast to WT-D, after 4 weeks of diabetes, KO-D mice did not develop the increase in inulin clearance (C(In)) characteristic of early diabetes. The nitric oxide synthase inhibitor N(omega)-nitro-l-arginine methylester suppressed C(In) in WT-D, but had no effect on C(In) in KO-D. Treatment of KO-D with the SOD mimetic tempol for 4 weeks suppressed albuminuria, increases in glomerular transforming growth factor beta, collagen alpha1(IV), nitrotyrosine, and glomerular superoxide, and concurrently increased C(In). The latter action of tempol in KO-D was blocked by the N(omega)-nitro-l-arginine methylester. The findings provide support for a role for superoxide and its metabolism by SOD1 in the pathogenesis of renal injury in diabetes in vivo, and implicate increased interaction of superoxide with nitric oxide as a pathogenetic factor.