Syzigium cumini, commonly known as 'jamun', is widely used in Indian folk medicine for the treatment of diabetes mellitus. Oral administration of 2.5 and 5.0 g/kg body weight of the aqueous extract of the seed for 6 weeks resulted in a significant reduction in blood glucose and an increase in total haemoglobin, but in the case of 7.5 g/kg body weight the effect was not significant. It also prevents decrease in body weight. The aqueous extract also resulted in decreased free radical formation in tissues studied. Thus the study shows that Jamun seed extract (JSEt) has hypoglycaemic action. The decrease in thiobarbituric acid reactive substances (TBARS) and increase in reduced glutathione (GSH), superoxide dismutase (SOD) and catalase (CAT) clearly show the antioxidant property of the JSEt. The effect of JSEt was most prominently seen in the case of animals given 5.0 g/kg body weight. JSEt was more effective than glibenclamide.