Genetically obese Zucker rats pair-fed to lean controls were similar in body weight and food intake, however, epididymal fat pads were considerably larger than lean controls. In vivo incorporation of acetate-14 C into adipose tissue lipid was not significantly different, however, in vivo liver lipogenesis was elevated in the obese rat. Characterization of enzyme profiles in both liver and adipose tissues revealed that enzymes normally associated with lipogenesis were elevated in liver tissue from obese rats. Malic enzyme and citrate cleavage enzyme were both depressed in adipose tissue of obese animals. From these data, it appears that the liver may be prominently involved in the development of excessive blood lipid and enlarged fat cells in the Zucker obese rat.