Guava is a plentiful fruit in Taiwan and it was taken from the plants of Psidium guajava Linn. (Myrtaceae). According to the folklore in Chinese Medicine, gauva was useful in the treatment of diabetes mellitus. In the present study, acute i.p. treatment with 1 g/kg guava juice produced a marked hypoglycemic action in normal and alloxan-treated diabetic mice. Although effective duration of guava is more transient and it is less potent than chlorpropamide and metformin, blood glucose lowering effect of guava also can be obtained by oral administration in maturity-onset diabetic and healthy volunteers. Thus, it is suggested that guava may be employed to improve and/or prevent the disease of diabetes mellitus.