The effect of single dose whole body irradiation on the serum cholesterol content and fatty acid composition of serum lipids of rats was investigated. A change in fatty acid composition of liver lipid was also observed. 1) The serum cholesterol content increased in parallel with the increase in body weight. The change in cholesterol ester was more marked than that of free cholesterol. 2) After 600 rad irradiation, cholesterol content increased, reached maximum 3 days after irradiation, and then decreased. This temporary increase was due to cholesterol ester. 3) Cholesterol and triglyceride both accounted for about 40 % , as measured by color determination on thin-layer chromatography. After irradiation, an increase in cholesterol content and the marked decrease in triglyceride content were observed bringing about a change in the amount of total serum lipid. 4) The fatty acid composition of normal and irradiated rat sera were compared. The relative percentages of total lipid of palmitic and oleic acids decreased while those of stearic and arachidonic acid increased. Serum phospholipid had little arachidonic acid and the fatty acid composition did not change after irradiation. Serum triglyceride had trace amounts of arachidonic acid and the unsaturated fatty acid component decreased after irradiation. On the other hand, unsaturated fatty acid in cholesterol ester increased after irradiation while linoleic and arachidonic acids made up 29 % and 22 % in control and 17 % and 61 % after irradiation, respectively. These results indicate that after irradiation the amount of oleic acid decreased in all lipid fractions, and that the decrease in linoleic acid and the increase in arachidonic acid after irradiation were due mainly to changes in the amounts of the cholesterol ester of these fatty acids. 5) Fatty acid composition of total liver lipid after irradiation showed a decrease in palmitic and oleic acids and an increase in stearic and arachidonic acids, the same trend as observed in serum lipid fatty acid. Liver cholesterol ester showed trace amounts of linoleic and arachidonic acids and an increase in short chain fatty acid after irradiation. 6) The major component of serum phospholipid was phosphatidylcholine, while palmitostearyl lecithine and unsatulated fatty acid were small components. Moreover, phosphatidylcholine and -ethanolamine were the major components of liver phospholipid, having highly unsatulated fatty acids. The changes in fatty acid composition were similar to the changes in total phospholipids.