Free radicals in olive oils were identified and quantified by EPR, by means of the spin-trapping technique making use of alpha-phenylbutylnitrone (PBN) as spin trap. The radical species were identified as PBN-trapped hydroxyl radicals (PBN-*OH) in the water microdroplets inside the fat medium. The largest radical concentration was 12.5 microM identical with 100%. The following were the relative concentrations of the radicals under different conditions: (1) Two oils, produced by continuous centrifugation, aged for 1 year, showed a 25-30% increase in the radicals compared to nonaged oils; 1-year-old oil, produced by pressure, did not differ from the nonaged oil. (2) Radical production was markedly reduced by N(2) bubbling; it was increased by heating, whereas it showed a biphasic pattern by air bubbling over time. (3) Radical concentration as a function of the UV irradiation time increased up to a maximum, after which it decreased and finally remained constant. The phenolic and oxygen contents were related to the radical content. This study demonstrates that the EPR technique is suitably applied to the detection of free radicals in olive oil and that storage, handling, and stress conditions of the oils significantly influence the radical concentration.