1. The effects of a high-fat, high-energy diet and essential plus semi-essential amino acid gavage on pup rats have been studied (60-65 animals). 2. The activities of alanine transaminase, adenylate deaminase, glutamine synthetase and serine dehydratase have been tested in liver and muscle. 3. Plasma was used for the estimation of proteins, urea, amino acids, glucose, lactate, 3-hydroxybutyrate and acetoacetate. 4. Liver and muscle glutamine synthetase activities are increased by diet and gavage administered. Hepatic serine dehydratase is inhibited by a cafeteria diet but activated by amino acid gavage. Adenylate deaminase is inhibited by diet and gavage in the liver, but gavage does not affect this enzyme activity in muscle. Liver alanine transaminase is increased by the diet; in the muscle, cafeteria diet and amino acid gavage showed the highest values for this enzyme. 5. In the plasma, the increase in lactate produced by the diet is inhibited by the amino acids provided. Cafeteria-fed pups showed lower urea levels and higher 3-hydroxybutyrate concentrations in the plasma. 6. Intracellular glucose is diminished by cafeteria diet. In contrast, the blood cell amino acid concentration increases with diet and gavage supplied.