Methanolic extract (75%) of Terminalia chebula, Terminalia belerica, Emblica officinalis and their combination named 'Triphala' (equal proportion of above three plant extracts) are being used extensively in Indian system of medicine. They were found to inhibit lipid peroxide formation and to scavenge hydroxyl and superoxide radicals in vitro. The concentration of plant extracts that inhibited 50% of lipid peroxidation induced with Fe(2+)/ascorbate were food to be 85.5, 27, 74 and 69 micro g/ml, respectively. The concentration needed for the inhibition of hydoxyl radical scavenging were 165, 71, 155.5 and 151 micro g/ml, and that for superoxide scavenging activity were found to be 20.5, 40.5, 6.5 and 12.5 micro g/ml, respectively. Oral administration of the extracts (100 mg/kg body weight) reduced the blood sugar level in normal and in alloxan (120 mg/kg) diabetic rats significantly within 4 h. Continued, daily administration of the drug produced a sustained effect.