The susceptibility of low-density lipoprotein (LDL) to oxidation was studied in hypertriglyceridemic men (5 with type III and 5 with type IV) at baseline on a low-saturated-fat, low-cholesterol diet, after 6 weeks of dietary supplementation with fish oil (Promega, 12 g/d), and after 6 weeks of fish oil combined with probucol (500 mg BID). The relative content of n-3 polyunsaturated fatty acids in plasma and LDL was increased during the two treatment periods, and a low a-tocopherol to n-3 polyunsaturated fatty acids ratio was observed. Plasma thiobarbituric acid-reactive substances (TBARS) levels were unchanged after 6 weeks of fish oil, but the ratio of lipid peroxides to the reduced triglyceride (TG) levels (MDA: TG) was significantly higher (p<.01). Addition of probucol lowered both absolute levels of TBARS (p<.01) and the MDA to TG ratio (p<.001). The susceptibility of LDL to Cu 2+ -catalyzed oxidation was evaluated over a 5-hour time course by determining TBARS formation, free amino group levels, and changes in LDL electrophoretic mobility. TBARS levels that were higher in native LDL (1.019«/< 1.050 g/mL) after 6 weeks offish oil than at baseline (p<.0l) were reduced 52.3±11.3% by the addition of probucol (p<.001). With fish oil alone, TBARS production after exposure of LDL to Cu 2+ for 5 hours was increased 17.0±5.8% compared with corresponding baseline values (p<.001), whereas a 64.1 ±14.3% reduction from the previous period was observed with fish oil+probucol p> <.001). Determination of LDL reactive amino groups further documented the structural changes occurring with peroxide formation, which were opposed by probucol. Our observations emphasize a potential risk associated with intake of large doses of fish oil in the treatment of hyperlipidemia and further demonstrate the ability of probucol as an antioxidant.