Fatty acid composition of various lipids isolated and fractionated from rat incisor pulps were analyzed by means of thin layer and gas chromatography. The changes of the compositions after whole body X-irradiation were also investigated. The results are as follows. 1) The fatty acid composition of total lipid from rat incisor pulps was similar to that of total lipids from rat incisor (hard tissue). 2) Palmitic, oleic, stearic and arachidonic acids, in that order, accounted for most of the fatty acids (>88 % ) in total lipid. In the irradiated group (at 3 days after whole body irradiation), oleic acid had decreased and arachidonic acid had increased. 3) The fatty acids of phospholipid were found in the order, palmitic, stearic, oleic and arachidonic acids. After irradiation, the percentage of oleic acid decreased and that of arachidonic acid increased. 4) Similarly, the fatty acids of triglyceride and cholesterol ester were in the order, palmitic, stearic, myristic, oleic and palmitoleic acid, and palmitic acid accounted for 60 % . Saturated fatty acids accounted for over 80 % of both lipid fractions and arachidonic acid was a trace component. After irradiation, the changes of fatty acid composition of both lipid fractions were different. A decrease in palmitoleic acid and an increase in myristic acid in the compositions of triglyceride were observed. On the other hand, a decrease in stearic and palmitoleic acids and an increase in myristic acid were observed in cholesterol ester. 5) The fatty acids of phosphatidyl-choline (a), ethanolamine (b) and serine (c) were in the orders; (a) palmitic, oleic, stearic and arachidonic acid; (b) stearic, oleic, palmitic and arachidonic acid; and (c) palmitic, oleic and stearic acid. Arachidonic acid in phosphatidyl choline and ethanolamine accounted for a high percentage of the composition, but was low in phosphatidyl serine. After irradiation, a decrease in oleic acid and an increase in palmitic and arachidonic acid in the compositions of phosphatidyl-choline and -ethanolamine were observed, whereas an increase in stearic and oleic acids and a decrease in palmitic acid was observed in the composition of phosphatidyl serine. 6) The contents of free and ester type cholesterol in the incisor pulps were determined. Total cholesterol content increased in parallel with increase in body weight, this increase could be accounted for by an increase in ester type. 7) After whole body X-irradiation, the content of total cholesterol increased temporarily and reached maximum 3 days after irradiation. This change of the content was due mainly to the change in the amount of ester type.