Abstract The effects of increasing levels of n-3 polyunsaturated fatty acids in fish oil on the composition of phospholipids in specific tissues and prostanoid biosynthesis important to vascular integrity were studied. Rats were fed menhaden oil at 5, 10 and 20 weight percent corresponding to 1, 2 and 4 percent dietary eicosapentaenoic acid (EPA). The control diets contained hydrogenated coconut oil and both diets included 2% safflower oil in order to be adequate in essential fatty acids. Significant differences in tissue selectivity for 20:5 and 22:6 were observed for platelet and aorta phospholipids. While platelets avidly took up 20:5 primarily at the expense of 20:4 n-6, 22:6 did not accumulate proprotionally in platelets even at the 20 percent dietary level. Alternatively, aorta phospholipids took up equal quantities of both 20:5 and 22:6 again at the expense of 20:4. Prostanoid biosynthesis by these tissues was significantly and selectively altered by dietary treatments. Whereas serum thromboxane levels were decreased by 50% in the group receiving 5% fish oil, prostacyclin production by isolated aorta segments from the same group was not correspondingly reduced. In contrast, the higher dietary levels: i.e. 10 and 20 percent fish oil, did not elicit corresponding reductions in thromboxane levels whereas prostacyclin production by isolated aorta tissue was increasingly affected and reached a 50% decrease at 20 percent dietary fish oil. These experiments demonstrated significant and perhaps important sensitivities by different tissues to dietary n-3 PUFA. In spite of these effects, the experimental animals did not exhibit a significant alteration in tail bleeding times, platelet aggregability or platelet sensitivity to the agonists ADP and collagen relative to control animals. Whole blood viscosity was however, lower in the group fed 20 percent fish oil.