Objective To investigate the protective effect of Quercetin on kidneys in diabetic rats. Methods STZ induced diabetic rats were given quercetin 100 mg?kg -1 ?d -1 for 8 weeks. Urinary albumin excretion rate (UAER) was measured by radioimmunoassay. The changes of creatinine clearance rate (Ccr) and glomerular protein kinase C (PKC) activities were determined. The expression of TGF ? 1 mRNA of renal cortex in diabetic rats were determined by RT PCR analysis. The glomerular changes were also observed morphologically. Results In untreated diabetic rats, Ccr, UAER, kidney weight/body weight and PKC activity in renal glomeruli were significantly increased, the expression of TGF ? 1 mRNA in renal cortex was elevated, and glomerular hypertrophy existed. After Quercetin treatment, Ccr, UAER, PKC activity and the expression of TGF ? 1 mRNA were markedly reduced as compared with those of untreated diabetic rats in 2 and 8 weeks, no significantly abnormal changes in kidney morphology were observed in Quercetin treated group. Conclusion Quercetin ameliorates early diabetic renal hyperdynamic abnormality via inhibiting PKC activity, in which inhibiting of TGF ? 1 production seems to be involved. Reduction of the PKC activity is important in preventing or delaying the development of diabetic nephropathy.