Oxysterols of non-enzymatic origin such as 7β-hydroxycholesterol are supposed to be involved in the pathogenesis of atherosclerosis. The aims of the present study were firstly to investigate whether in vivo oxysterol concentrations are affected by dietary treatment with fish oil, vitamin E, iron and oxidized cholesterol and secondly to explore physiological functions of diet derived oxysterols. Therefore, 5 experiments with male Sprague-Dawley rats were performed, in which the treatment factors fat, iron supplement, cholesterol, and vitamin E were varied. It was shown by the experiments 1, 2 and 3 that dietary administration of fish oil increased hepatic concentrations of non-enzymatically formed oxysterols such as 7β-hydroxycholesterol irrespective of the dietary vitamin E supply as compared to dietary lard or coconut oil. In contrast, concentrations of 7β-hydroxycholesterol in plasma, LDL and erythrocytes were only elevated in response to dietary fish oil as compared to dietary coconut oil when the vitamin E supply was insufficient. High iron diets increased concentrations of non-enzymatically formed oxysterols only when salmon oil was used as dietary fat. Insufficient dietary vitamin E increased in vivo concentrations of 7β-hydroxycholesterol, whereas a supply in excess of the dietary vitamin E requirement had no effect. Feeding oxidized cholesterol resulted in increased concentrations of 7β-hydroxycholesterol in liver, plasma, and LDL. It was shown in experiment 4 that dietary oxidized cholesterol increased the activity of hepatic glutathione peroxidase and decreased concentrations of glutathione in the liver irrespective of the type of dietary fat. In addition, mRNA concentrations of glutathione peroxidase and superoxid dismutase were increased by dietary oxidized cholesterol. The experiment 5 revealed that lipid concentrations in plasma and VLDL were lowered by oxidized cholesterol - an effect that is possibly related to the lowered gene expression of microsomal triglycerol transfer protein as observed in the present study. The amounts of arachidonic acid in hepatic triacylglycerol and phospholipids were increased by dietary oxidized cholesterol. To summarize, the results of the present work showed that the concentrations of pathophysiologic relevant oxysterols in the organism may be unfavourably influenced by the diet composition. Therefore, the observed physiologic effects of oxidized cholesterol - in particular the induction of oxidative stress, but also the altered membrane fluidity caused by the incorporation of oxysterols into the membranes - are possibly related to the multiple reported pathophysiologic effects of oxysterols and oxidized cholesterol, respectively.