The antioxidant effects on cod liver oil (CLO) of chlorogenic acid (CGA) and its metabolites, caffeic acid (CA) and quinic acid (QA) were investigated. All three chemicals were separately added to CLO at a concentration range of 0.001-0.080 wt-% and tested by Rancimat. CA was the best antioxidant; its antioxidant effect was directly proportional to the dose added, but only within the concentration range of 0.005-0.050 wt-%. QA did not show any effect on CLO oxidation, while CGA revealed itself as a weak antioxidant. Cytotoxicity was also evaluated by using murine 3T6 fibroblasts treated for 24 h with CGA, CA and QA at a concentration range of 0.1-5.0 mM.. QA did not have an influence on cell proliferation, while CGA and CA showed a significant anti-proliferative activity already at a concentration of 0.1 mM. Finally., compounds similar to the standard 8-iso prostaglandin 17(2 alpha), called CLO-"isoprostanes"-like compounds, were isolated. At high temperature (100 and 110 degrees C) CLO-"isoprostanes-like" Compounds did not show a relevant pro-oxidant action on the refined olive oil to which they were added in variable amounts. On the contrary, the CLO-"isoprostanes"-like compounds significantly decreased the antioxidant effectiveness of CA. Indeed, at a CLO-"isoprostanes-like" compound dose of 300 mu g, the antioxidant effectiveness of CA was halved.