The reactions of fatty acid biosynthesis from acetyl CoA in higher organisms are catalyzed by acetyl CoA carboxylase and a fatty acid synthetase multienzyme complex (Wakil, 19 63; Vagelos, 1964). Evidence from many laboratories (Numa et al--*, 1961; Martin and Vagelos, 1962; Waite and Wakil, 1962; Kallen and Lowenstein, 1962) indicates that regulation of fatty acid synthesis occurs at the level of acetyl CoA carboxylase. The carboxylase is activated by citrate and even when fully activated is alleged to be rate-limiting in fatty acid synthesis (Ganguly, 1960; Numa & a., 1961). However, no condition has been reported in which changes in carboxylase level per se account fully for changes in the rate of fatty acid synthesis. Hepatic fatty acid synthesis is 6-8 times higher in the genetically obese hyperglycemic mouse than in normal mice (Jansen fi al., 1967).

The initial objective of this investigation was to determine whether hepatic carboxylase and/or synthetase levels are increased commensurate with the elevated rate of fatty acid biosynthesis in obese mice. Both hepatic acetyl CoA carboxylase and fatty acid synthetase levels were found to be elevated to about