The antioxidant activity of extracts of evening primrose seeds (SE) and a commercially extracted filter cake (FC) were determined. The SE and FC were extracted with methanol/water (9:1) followed by evaporation and concentration. Extracts were tested in a bulk oil system and an oil-in-water emulsion using safflower oil as the major source of lipids. The antioxidant activity of the extracts was compared to that of a control and to that of butylated hydroxytoluene (BHT), singly, and in combination. Antioxidant activity was measured by the co-oxidation of beta-carotene, an oxidative stability instrument, conjugated dienes, and headspace analysis of hexanal. The SE extract had greater antioxidant activity than the FC extract. The SE extract was more effective in controlling the oxidation in the oil-in-water model system than in the bulk oil system. The activity of SE was concentration dependent, and at higher concentrations the SE was as effective as BHT, but it required higher concentrations because of its lack of purity. Synergism between SE and BHT was demonstrated in both model systems.