Tissue accumulation of sorbitol secondary to enhanced polyol-pathway activity is believed to play an important role in the development of diabetic complications. We previously demonstrated sorbitol accumulation, due in part to enhanced expression of aldose reductase (AR) in the diabetic kidney. In this study, we quantitated AR enzyme activity, immunoreactivity, and mRNA in various tissues from nondiabetic and diabetic BB/Wor rats 3 mo after onset of diabetes. In addition, the effects of intensive insulin treatment (3-6 U/day) and the effects of the AR inhibitor Statil (25 mg.kg-1.day-1) on AR expression were determined. Of 13 tissues examined, AR activity was significantly increased in the lens, kidney, sciatic nerve, skeletal muscle, retina, and spinal cord from diabetic rats compared with age-matched nondiabetic control rats. In most tissues, AR immunoreactivity and AR mRNA were proportionately elevated. Intensive insulin treatment, which normalized blood glucose and glycosylated hemoglobin, significantly reduced AR activity and immunoreactivity. AR mRNA abundance was also reduced in tissues from insulin-treated diabetic rats. Statil treatment had no significant effect on AR immunoreactivity or AR mRNA abundance, although AR activity in tissues from Statil-treated diabetic rats was significantly reduced compared with untreated diabetic rats. These studies demonstrate that the expression of the AR gene is upregulated in most tissues of the diabetic rat, that insulin treatment reverses this phenomenon, and that AR inhibition has no effect on AR gene expression.