The fatty acid composition of five normal tissues and purified liver plasma membranes of the mouse was modified by feeding diets rich in either coconut or sunflower oil. The phospholipids from the animals fed the coconut oil diets were enriched in monoenoics, and those from the mice fed the sunflower oil diet contained large percentages of polyenoics. Most importantly, there were differences in the extent and type of modification in each tissue. For example, the phospholipids of the heart from animals fed the sunflower oil diet contained 21 % docosapentaenoate compared to less than 6 % in other tissues. The bone marrow from animals fed the coconut oil diets contained a high proportion of saturates, including those of medium chain length. The fatty acyl composition of the neutral lipids was also modified, but in some cases the degree of alteration differed from that occurring in the phospholipids. Purified liver plasma membranes demonstrated diet-induced changes but contained smaller amounts of arachidonate, and those from the animals fed sunflower oil diets were less poly-unsaturated compared to whole liver phospholipids. We conclude that the modification of fatty acid composition of normal tissues induced by experimental diets differs in extent and type. Furthermore, the liver plasma membrane composition is different from that of liver phospholipids. The differences in the extent and type of modification demonstrate selectivity amongst normal tissues.