Rabbits feeding on atherogenic diet for 60 days resulted in high level of plasma lipid peroxides as well as extreme hypercholesterolemia. Both of them kept at high level until 35 days after atherogenic diet stopped. At the same time, as compared with the control group, plasma PGI2 level was remarkably decreased while TXA2 and platelet aggregability were increased. Atherosclerotic vessel walls contain high levels of lipid peroxides associated with decreased PGI2 and increased TXA2 generation. Atherosclerotic plaques had the highest level of lipid peroxides and TXA2 while PGI production was the least, as compared with non-plaque tissue of the same artery and the normal arteries. The condition of normal arteries was just reverse. It was concluded that the concurrent presence of lipid peroxidation products may be seriously considered when evaluating the hyperlipidemia as a cause of atherosclerosis, and the elevation of TXA2 generation in arteries might be taken into consideration when evaluating an imbalance of PGI2/TXA2 in plasma.