The hypoglycemic and anti-diabetic effect of Rehmannia glutinosa oligosaccharide (ROS) in glucose-induced hyperglycemic and alloxan-induced diabetic rats and its mechanism was investigated in this paper. It was found that pretreatment of ROS in normal rats with 100 mg/kg for 3 days, i.p., induced a partial prevention of hyperglycemia caused by glucose (2g/kg, i.p.), while when hyperglycemia was induced in adrenalectomized (ADX) rats, the preventive effect of ROS on hyperglycemia was lost. In alloxan-induced diabetic rats, ROS (100 mg/kg for 15 days, i.p.) showed a significant decrease in blood glucose level and hepatic glucose-6-phosphatase activity with an increase in hepatic glycogen content. Furthermore, ROS raised plasma insulin level and lowered plasma corticosterone level in alloxan-induced diabetic rats. The results indicated that oligosaccharide of Rehmannia glutinosa Libosch. exerted a significant hypoglycemic effect in normal and alloxan-induced diabetic rats. The regulatory mechanism of ROS on glucose metabolism was adrenal dependent and had a close relation with the neuroendocrine system.