Oxidative modifications of apo-B-containing lipoproteins (LDL+VLDL) appear to play a role in atherogenesis. Increased atherosclerosis is one of the major causes of morbidity and mortality during ageing. The aim of this study was to investigate the susceptibility of serum and LDL+VLDL to lipid peroxidation in 12 young (6 months) and 12 old (22 months) rats. Serum endogenous malondialdehyde (MDA) and diene conjugate (DC) levels were found to be increased by 24.9% and 30.0% respectively, in old rats. In addition, 2,2'-azobis-(2-amidino-propane) hydrochloride (AAPH)-induced lipid peroxide levels were increased in serum of old rats. Although serum vitamin E levels were significantly increased (27.4%), there was a significant decrease in cholesterol-adjusted vitamin E levels (14.3%) in old rats. Serum vitamin C and total sulphydryl levels were found to be decreased by 25.5% and 22.7% respectively. The ferric reducing ability of plasma (FRAP) was also lower (15.9%) in old rats. Endogenous DC and copper-induced MDA levels were significantly higher (65.1% and 26.7% respectively) in LDL+VLDL fractions obtained from EDTA-plasma by dextrane sulphate and MgCl2 solution in old rats. The propagation rate and maximal amount of DC increased, but the lag phase and t(max) were shortened in LDL+VLDL fractions of old rats. Our results clearly indicate that the susceptibility of whole serum and LDL+VLDL fractions to lipid peroxidation is increased in aged rats.