An aqueous extract of Casearia esculenta was found to lower blood glucose in basal conditions and after a glucose load in normal rats. Maximum reduction in blood glucose was observed between 2-3 h at a dose level of 200 and 300 mg/kg body weight. C. esculenta extract was also found to reduce the blood sugar level in streptozotocin--induced diabetic rats. Oral administration of the extract significantly reduced the blood sugar in streptozotocin induced diabetic rats for 15 days. The extract was also found to reduce the increased plasma thiobarbituric acid reactive substances (TBARS), blood urea and improvement in body weight reduction induced by streptozotocin injection. These results indicate that C. esculenta extracts are able to ameliorate biochemical changes induced by streptozotocin in diabetic rats.