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Non-fa/fa and fa/fa rats were weaned at 18 days of age and fed either a high fat or high carbohydrate diet. At 6 weeks of age some of the rats fed high fat were fed the high carbohydrate diet until 10 weeks of age. Food consumption was equalized between all treatment groups. Epididymal fat pads were heavier in the fa/fa rats in all treatment groups. Fat pad weight and cellularity increased in the non-fa/fa rats fed the high fat diet. Cellularity was actually less in the fa/fa than in the non-fa/fa rats, but high fat feeding resulted in higher proportions of larger adipocytes in both phenotypes. Rates of epididymal fat pad lipogenesis and enzymatic activities were similar in both non-fa/fa and fa/fa rats in all treatment groups. Hepatic fatty acid synthesis and enzymatic activities were consistently greater in the fa/fa rats. Carbohydrate feeding resulted in increased hepatic fatty acid synthesis regardless of the earlier feeding of the fat diet, but the fa/fa rats were more responsive to the carbohydrate diet than the non-fa/fa rats. A delay in introduction of carbohydrate to the diet did not alter the eventual lipogenic capacity of fa/fa rats. Lipogenesis appeared responsive to the most recent diet.