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