This work proposes an oxidative stress test based on the measurement of the heat generated during the reaction of AIBN (2,2'-azobis(2-methylpropionitrile)) with lipophilic antioxidants. Without antioxidants, AIBN generates an exothermic peak (induction period = 7.50 h, peak time = 8.55 h, area = 3.6 kJ mol(-1) of AIBN, at 50 degrees C). In the presence of antioxidants, such peak is delayed. The extent of such delay provides a simple and direct estimate of the radical-scavenging activity of the sample. Standard solutions of well-known antioxidants lead to the following ranking (from high to low radical scavenging activity): ethoxyquin > > (+/-)-alpha-tocopherol > butylated hydroxytoluene (BHT) > retinyl acetate, in close agreement with the 2,2-diphenyl-L-picrylhydrazyl (DPPH) assay. The proposed assay was applied to characterize the radical-scavenging activity of five extra-virgin olive oil samples. The results were in good agreement with the total phenol content of each sample (R-2 = 0.975).