Autoxidation of corn and soybean oils was prevented in the presence of 0.1 g/kg rosemary (Rosemarinus officinalis L.) extract and butylated hydroxy toluene (BHT). At the end of a 4-d period, the peroxide value in corn oil reached 13.0 and 8.2 in the presence of rosemary extract and BHT respectively. For soybean oil, the peroxide value was found to be 10.0 and 6.6 in the presence of rosemary extract and BHT respectively Rosemary extract and BHT when added as mixtures of 75:25, 50:50 and 25:75 had a synergistic effect on preventing oxidation of soybean oil. Superoxide radicals (O-2(.-)) were generated by phenazine methosulphate-NADH system and rosemary extract was found to scavenge O-2(.-) in a concentration-dependent manner. The antioxidative effect observed is believed to be partly due to this radical scavenger activity. (C) 1997 Academic Press Limited.