We investigated the effects of garlic oil and diallyl disulfide (DADS) on glycemic control and renal function in rats with streptozotocin-induced diabetes. Rats received by gavage garlic oil (100 mg/kg body wt) or DADS (40 or 80 mg/kg body wt) every other day until 16 weeks after the induction of diabetes. The control rats were treated with corn oil only. Neither garlic oil nor DADS significantly affected fasting blood glucose concentrations throughout the investigation period. Garlic oil did not affect oral glucose tolerance in diabetes acutely but significantly improved oral glucose tolerance at 4, 8, 12, and 16 weeks and significantly ameliorated proteinuria at the end of 16 weeks. DADS did not significantly affect oral glucose tolerance or renal function. Diabetic rats fed 80 mg DADS/kg body wt had a significantly lower rate of body weight gain and a significantly lower ratio of muscle weight to body weight than did vehicle-treated diabetic rats. In conclusion, long-term treatment of diabetes with garlic oil can improve oral glucose tolerance and renal function in diabetes but not through the action of DADS. High doses of DADS may further complicate the metabolic disturbances in diabetes.