The methanol extract of seeds of Swietenia macrophylla King. (MESM) was studied for its antidiabetic activity in streptozotocin induced diabetic rats. It was principally aimed to correlate the efficacious role of MESM on reduction of oxidative state associated with diabetes. The extract was found to be potent antidiabetic evidenced by significant reduction of blood glucose level in diabetic rats (47.96% reduction of blood glucose level, at 300 mg/kg, on day 10). It was found that, MESM at 300 mg/kg, significantly decreased TBARS (35.03 and 22.22%) whilst increased GSH (86.75 and 31.45%), SOD (93.05 and 45.88%) and CAT (56.99 and 68.46%) levels in liver and kidney respectively in diabetic rats.