Chlorella, a type of unicellular fresh water algae, has been a popular foodstuff in Japan and Taiwan. Chlorella has been shown to produce hypoglycemic effects in alloxan-induced diabetic animals. However, there are no other reports of the effects of this substance in other diabetic animal models. Here we have used streptozocin (STZ)-induced diabetic mice to study the thypoglycemic effects of Chlorella. Diabetes was induced in ICR strain mice by the i.p. injection of STZ. Vehicle-treated ICR mice were used as normal control animals and glibenclamide was used as a positive drug control. The effects of Chlorella on basal blood glucose, exogenous insulin sensitivity test and plasma insulin levels were measured. In normal mice Chlorella produced a transient hypoglycemic effect at 90 min after acute administration; whereas glibenclamide produced a more sustained hypoglycemic effect between 90 min and 180 min after acute administration. Chlorella did not affect the basal blood glucose level in STZ mice. However, Chlorella enhanced and prolonged the hypoglycemic effects of injected insulin in STZ mice for a further 60 min compared to the normal vehicle-treated group. Plasma insulin levels were increased in normal mice after treatment with glibenclamide, whereas Chlorella had no such effect. The current results indicate that Chlorella enhances the hypoglycemic effects of exogenous insulin at a dose which does not produce hypoglycemia in STZ mice, suggesting that insulin sensitivity is increased in these mice.