The antidiabetic potential of the alcoholic stem extract of Coscinium fenestratum Colebr. (Menispermaceae), a medicinal plant widely used in the traditional Ayurveda and Siddha systems of medicine for the treatment of diabetes mellitus was evaluated in the STZ-nicotinamide induced type 2 diabetic model. Graded doses of the alcoholic stem extract were administered to normal and experimental diabetic rats for 12 days. Significant (p <0.05) reduction in fasting blood glucose levels were observed in the normal as well as in the treated diabetic animals. Serum insulin levels were not stimulated in the animals treated with the extract. In addition, changes in body weight, serum lipid profiles, thiobarbituric acid reactive substance levels, glycosylated hemoglobin and liver glycogen levels assessed in the extract treated diabetic rats were compared with diabetic control and normal animals. Significant results were observed in the estimated parameters, thereby justifying the use of the plant in the indigenous system of medicine. © 2004 Elsevier Ireland Ltd. All rights reserved.