The methanol extract of the seed of Swietenia macrophylla (Meliaceae) seeds were investigated for its hypoglycaemic activity in normal and alloxan induced diabetic rats. Seed extract suspended in aqueous Tween 80 solution, the graded doses were administered to normal and experimental diabetic rats for 10 days. Fasting blood glucose levels, serum lipid profiles and changes in body weight were evaluated in normal rats while the hepatic marker enzymes and liver glycogen levels were evaluated additionally in diabetic rats. The diabetic groups treated with the seed extract were compared with standard drug glibenclamide. The findings showed the significant antidiabetic potential of the extract in ameliorating the diabetic conditions in diabetic rats. The extract also has hypoglycaemic action in normal rats. A toxicity study has been performed for the extract, which revealed that the extract is safe to use even at the doses of 3.2 mg/kg of body weight orally. The result from the present investigation indicates good correlation between the folk use and scientific investigation.