In this study, we examined the effects of a" Western-type" diet containing 9%(w/w) fat and 0.15%(w/w) cholesterol, in the presence or absence of 2%(w/w) phytosterol mixture over an 18-week period in apolipoprotein E-deficient mice. Addition of phytosterols to the high cholesterol diet was associated with normalization of the depressed hepatic 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity (from 22.3+/-6.3 to 55.4+/-19.9 pmol/mg protein/minutes, p< 0.05). This finding was associated with a significant decrease in plasma and hepatic cholesterol concentrations compared with animals fed the high cholesterol diet without phytosterols (33.3+/-5.0 versus 19.2+/-6.2 pmol/mg protein, p< 0.05). The activities of cholesterol 7alpha-hydroxylase and sterol 27-hydroxylase were comparable between the two groups of mice. Urinalyses and hematologic data were comparable between the two groups except for significantly lower platelet counts in the phytosterol-treated animals (681.6+/-118.9 versus 857.1+/-185.4 x10 (9)/L, p< 0.05). The phytosterol-treated animals had significantly (p< 0.05) less fragile erythrocytes when exposed to 0.08, 0.07, or 0.05 M NaCl compared with cholesterol-fed mice. The consumption of the Western-type diet was associated with the development of xanthomatous skin lesions in 33% of the cholesterol-fed animals, but in none of the phytosterol-treated animals. Histologic examination revealed oil red O-negative vacuolation in liver and kidney parenchymal cells of the cholesterol-fed group, but not in the phytosterol-treated mice. Arrested spermatogenesis and atrophy of seminiferous tubules were observed, to a variable extent, in both groups of animals. We conclude that addition of the phytosterol mixture (2% w/w) to a Western-type diet in apolipoprotein E-deficient mice significantly decreases plasma and hepatic cholesterol concentrations, increases hepatic 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity, and prevents cutaneous xanthomatosis and vacuolation in the parenchymal cells of kidneys and livers.