The present study was conducted in order to examine the effect of acarbose, a potent alpha-glucosidase inhibitor, on renal function in rats with mild streptozotocin-diabetes. Male Wistar rats were made mildly diabetic by intravenous injection of streptozotocin (40 mg/kg) and were supplied a standard solid chow containing 0.1% acarbose for 8 weeks. Diabetic rats showed mild hyperglycemia under non-fasting condition and their urine albumin excretion (UAE) rate was markedly increased compared to non-diabetic control rats, while acarbose treatment resulted in a significant suppression of blood glucose level and UAE in diabetic rats. Examination by electron microscope revealed that the number of anionic sites in the lamina rara externa per 1000 nm of glomerular basement membrane (GBM) was significantly decreased in diabetic rats compared to control value (15.7 +/- 0.9 vs. 20.9 +/- 0.3 P < 0.001), whereas, significant recovery (19.6 +/- 0.6 P < 0.01) was observed after 8 weeks of acarbose treatment. In conclusion, acarbose treatment suppressed blood glucose level of mildly-insulin deficient animal model without insulin treatment and prevented from a reduction in the number of anionic sites in GBM which might ameliorate an increased permeability of GBM leading to albuminuria.