Onion (Allium cepa Linn) has been reported to have hypoglycemic activity in human and several animal models. In the present study, we performed intraperitoneal glucose tolerance test (IPGTT) in young (1.5mo) and aged (5 mo) rats treated with onion in order to determine whether aging can influence on the anti-hy-perglycemic effect of onion. In addition, we investigated the hypoglycemic effect of onion in streptozotocin- induced diabetic rats treated with aqueous extracts of onion (500 mg/kg, i.p., daily) for 4 weeks. Blood glucose level was determined in fasted and fed rats by using a glucometer (Johnson & Johnson). In glucose tolerance test, blood glucose level was maximally increased 15 min after glucose load (2 g/kg, i.p.), and recovered to the basal level 3 hr after glucose challenge in young and old rats. The maximum blood glucose levels of young and aged rat were 184$pm$7.49 and 225.2$pm$ 12.55 mg/dl, respectively. A single i.p. injection of aqueous extract of onion (1 g/kg) 30 min before glucose challenge significantly decreased blood glucose levels at 15, 30, 60, 90 min after glucose load in aged rats, while the administration of onion did not show any significant effect in young rats. In onion-treated diabetic rats, significant hypoglycemic effect (p<0.05) was observed, and the effect was greater in fasted rats than in fed. In conclusion, these results suggest that anti-hyperlycemic effect of onion can be changed by age and fasting.