In 30 rabbits, intraven injection of thyroxine produced characteristic changes in the lipids of different tissues; the phospholipid fatty acid content of liver decreased, while the non-phospholipid fatty acid content increased; in skeletal muscle, reverse changes occurred[long dash]phospholipid fatty acids increased and non-phospholipid fatty acids decreased; in blood, both phospholipid and non-phospholipid fatty acids increased. The increase in muscle phospholipids agrees with the idea that tissue phospholipid content parallels metabolic activity. However, liver phospholipid decreases can not be explained on such a basis. It is possible to explain both results by assuming that phospholipids of both tissues participate in fat combustion.