1. For the purpose to clarify the relationship between the structural change and lipid composition of isolated rat liver mitochondria, lipid composition and swelling rate of mitochondria obtained from the rat of 3'-Me-DAB feeding and raised in cold room are measured, and the following results were obtained. 2. The mitochondria obtained from the liver of 3'-Me-DAB-fed rat and of rat raised in cold room show a low rate of swelling by addition of Na-oleate accompanied by the decrease in highly unsaturated fatty acids (C18:3 and C20:3or 4) and with the increase in saturated fatty acids (C16 and C18). 3. Activation energy for the mitochondrial swelling is about 16.2 Kcal in the mitochondria obtained from normal rat liver, but requires 19.7 Kcal in the mitochondria that show a low rate of swelling. The fatty acid composition,
especially in glycerophosphatides which occupy about 80 per cent of total lipids, is a structural component of mitochondrial membrane, undergoes the change from former to latter in the following fashion: C16:0 21.73→32.10, C16:1 3.37→2.96, C18:0 25.0→29.75, C18:1 13.75→17.40, C18:2 23.90→16.0 and C20:3 or 4 12.23→1.79. 4. At the time of low rate swelling of mitochondria isolated from 3'-MeDAB- fed rat liver, there could be observed a marked increase of the acetone soluble lipid (simple lipids) in the total liver lipids and in the fatty acid distribution of the acetone-soluble lipids, oleic acid was markedly increased (0.838→3.81%/dry liver), despite the fact that in the acetone-insoluble fractions or in
the mitochondria there are no marked changes in the oleic acid contents (1.84→2.56% or 0.212→0.246%/dry liver).</p
