BHE and Wistar male, weanling rats were fed either a 65% sucrose or a 65% cornstarch diet. They were killed at 50 d of age; livers were excised and used for the determination of the fatty acids of mitochondrial phospholipid or used to determine the effect of such diets on the dependence on temperature of succinate-supported respiration. Neither diet nor strain affected either the total quantity of phospholipids or the quantities of the individual phospholipids. However, analysis of variance revealed strain and diet differences in the fatty acyl moieties of the phospholipids: sucrose-fed rats had more stearic (18:0) and less linoleic acid (18:2) than did starch-fed rats; BHE rats had more arachidonic acid (20:4) than did Wistar rats. There were no significant differences in either transition temperature or in the lower or upper activation energies determined from the biphasic Arrhenius plot. Although fatty acid compositional differences were observed, these differences had little effect on the membrane-associated, succinate-supported respiration.