Increased accumulation of renal sorbitol has been documented in the diabetic rat, and it has been suggested that this accumulation may be important in the pathogenesis of diabetic nephropathy. It is not clear whether sorbitol accumulation results from increases in substrate, activity of the aldose reductase (AR) protein molecule, or activity due to an increase in the amount of enzyme present. In this study, we have quantitated renal AR activity, immunoreactivity, and mRNA in rats 3 mo after induction of diabetes with streptozocin (STZ-D, 65 mg/kg body wt). Renal AR activity was significantly increased in diabetic rats compared with age-matched nondiabetic controls (0.95 +/- 0.05 vs. 0.51 +/- 0.03 U.mg-1.h-1, respectively, P less than .0005). Western blot analysis demonstrated that the antiserums recognized a single 40,000-Mr protein species in renal homogenates from both diabetic and nondiabetic rats. When quantitated in an immunodot assay, AR immunoreactivity was significantly increased in diabetic rats compared with nondiabetic controls (0.57 +/- 0.03 vs. 0.33 +/- 0.02 U, respectively, P less than .0005). Hybridization of Northern blots with a synthetic 36-nucleotide oligomer and an AR cDNA identified a 1.4-kilobase pair transcript; the abundance of the transcript was significantly increased in poly(A)+ RNA from the kidneys of diabetic compared with nondiabetic rats (P less than .005). This study demonstrates that renal AR activity is increased in the STZ-D rats and suggests that the increased AR activity can be in part explained by enhanced AR gene expression.