Probucol is a powerful inhibitor of atherosclerosis in a number of animal models. However, it is unknown whether this is due to the strong antioxidant protection of low density lipoprotein (LDL), to antioxidant effects in the artery wall, or to cellular effects not shared by other antioxidants. To investigate whether murine models are suitable to study the antiatherogenic mechanisms of probucol, three experiments following different protocols were carried out in 135 male and female LDL receptor-deficient (LDLR-/-) mice. Treatment groups received a high (0.5%) or low (0.025%) dose of probucol, or low-dose probucol plus a high dose (0.1%) of vitamin E for periods ranging from 6 to 26 weeks. In all experiments, probucol strongly protected LDL against ex vivo oxidation (lag times exceeding 1400 min in 0.5% probucol-treated mice). Treatment with 0.5% probucol significantly lowered both HDL-cholesterol and plasma apolipoprotein (apo)A-I concentrations. In all three experiments, treatment with 0.5% probucol consistently increased the size of lesions in the aortic origin, from 1.3-fold (n.s.) to 2.9-fold (P < 0.05) in female mice and from 3.6- to 3.7-fold in males (P < 0.001). Even treatment with 0.025% probucol increased atherosclerosis 1.6-fold in male mice (P < 0.01). Addition of the high dose of vitamin E did not attenuate the pro-atherogenic effect of 0.025% probucol. In conclusion, probucol not only failed to decrease but actively increased atherogenesis in LDLR-/- mice in a dose-dependent manner, even though it provided a very strong antioxidant protection of LDL. This suggests that the reduction of atherosclerosis observed in other animal models is due to intracellular effects of probucol not found in mice, to differences in the metabolism of probucol, and/or to an overriding atherogenic effect of the decrease in HDL in murine models.