The effect of varying n-6/n-3 ratios (0.6-10.2) of dietary fats on various lipid parameters was examined in rats under a constant P/S ratio (1.4-1.5) with sardine oil as the source of n-3 polyunsaturated fatty acid (PUFA) by a combination of palm and safflower oils. The concentration of serum cholesterol tended to increase with n-6/n-3 of up to approximately 2, whereas aortic cholesterol decreased. The proportion of arachidonic acid in liver, heart, and aorta phosphatidylcholine increased linearly with increasing n-6/n-3 whereas that of linoleic acid reached a plateau at this ratio of approximately 4. The proportion of n-3 PUFAs decreased with increasing n-6/n-3 in tissue phosphatidylcholine. Although the production of prostacyclin (PGI2) by the thoracic aorta and thromboxane A2 (TXA2) by platelets increased with increasing n-6/n-3, TXA2/PGI2 was maintained at a low level up to n-6/n-3 of approximately 5. These results indicate that, when fish oil is the source of n-3 PUFAs, n-6/n-3 of 2-5 seems to be desirable for the various lipid parameters related to atherosclerosis and thrombosis.