The hypoglycaemic effect of the aqueous extract of the bark of Helicteres isora L. (Sterculiaceae) was investigated in normal, glucose load conditions and streptozotocin (STZ)-induced diabetic rats. In normal rats, the aqueous extract of the bark of Helicteres isora L. (100 and 200 mg/kg/p.o.) significantly (P<0.001) reduced the blood glucose levels from 64.5-48.5 and 67-47 mg% 2h after oral administration of bark extract and also significantly lowered the blood glucose in STZ diabetic rats from 68-105 and 66-85.5 mg% 21 days after daily oral administration of the extract (P<0.001). The results suggested that the aqueous extract of bark of Helicteres isora L. possesses a potential hypoglycaemic effect in diabetic rats.