The effect of ethanolic extract of dried nuts of Semecarpus anacardium on blood glucose level was investigated in both normal and alloxan induced diabetic rats. The blood glucose levels were measured at 0, 1, 2 and 3 hours after the treatment. The ethanolic extract of S. anacardium (100 mg/kg) reduced the blood glucose of normal rat from 84 +/- 1.4 to 67 +/- 1.7 mg/dl, 3 hours after oral administration of the extract (P < 0.05). It also significantly lowered blood glucose level in alloxan induced diabetic rat from 325 +/- 2.2 to 144 +/- 1.4 mg/dl, 3 hours after oral administration of the extract (P < 0.05). The antihyperglycemic activity of S. anacardium was compared with tolbutamide, an oral hypoglycemic agent.