The inhibitory effects were examined of a hot-water extract of coffee beans on Kud: Wistar rats by an in vivo oral saccharinity tolerance test (OST), and the in vitro suppression of alpha-amylase and alpha-glucosidase activity. The hot-water extract of coffee beans strongly inhibited the activities of alpha-amylase and alpha-glucosidase, and reduced the postprandial blood glucose concentration by OST. Chlorogenic acid and acarbose strongly inhibited the activity of alpha-glucosidase, and reduced the postprandial blood glucose concentration by OST. However, caffeine, which is a major component of coffee beans, had no such effect. These results suggest that the inhibitory effect of the hot-water extract of coffee beans on postprandial hyperglycemia may have been due in part to the inhibition of alpha-glucosidase by chlorogenic acid which is a major component of coffee beans.