This study was conducted to determine the effects of a low polyunsaturated fatty acid (PUFA) 21 diet versus a high-PUFA diet on lipid peroxidation and on low density and very low density lipoprotein (LDL + VLDL) oxidation in vivo. Rats were fed 10% beef tallow (BT) or 10% soybean oil (SO) diet for 21 weeks. Lipid peroxidation was measured by assessing urinary 24 excretions of secondary lipid peroxidation products, by HPLC and by measuring thiobarbituric acid reactive substances (TBARS) in liver tissue. Plasma LDL + VLDL in vivo oxidation was measured by conjugated diene concentration and TBARS; ex vivo resistance to copper-induced oxidation was also assessed. Total urinary aldehydes, twelve individual urinary aldehydes, and TBARS in the liver were significantly lower in the BT group compared to the SO group. Plasma LDL + VLDL was significantly more resistant to copper-induced ex vivo oxidation to the BT group compared to the SO group. However, in vivo plasma LDL + VLDL oxidation levels measured as conjugated dienes and by TBARS were not significantly different. In general, the low-PUFA BT diet appears to have a protective effect on in vivo lipid peroxidation compared to the high-PUFA diet, but not on in vivo plasma LDL + VLDL oxidation.