The effect of allatectomy on the changes in activity of enzymes concerned with the conversion of dietary carbohydrate to lipid by the fat body during development has been investigated. The enzymes studied were phosphofructokinase, pyruvate kinase, malic enzyme, glucose-6-phosphate dehydrogenase isocitrate dehydrogenase, ATP citrate lyase, and fructose-1,6-diphosphatase. All operations were carried out at 4 days after emergence and allatectomized insects were compared with sham operated controls. The sham-operated controls showed similar enzyme changes to normal insects during development, thus the physical stress of the operation does not affect fat body metabolism. The specific activities of pyruvate kinase and phosphofructokinase were lower in the fat body samples from the allatectomized insects, whereas the specific activities of the other enzymes were all higher. Changes in the total amount of these enzymes show that ATP citrate lyase increases in parallel with the increased conversion of carbohydrate to lipid. The amounts of glucose-6-phosphate dehydrogenase and isocitrate dehydrogenase are also increased. These changes are consistent with the rôle of these enzymes in lipogenesis. Pyruvate kinase and phosphofructokinase show relatively little change which correlates well with the measurement of total glucose metabolism. Malic enzyme changes in a similar manner to the glycolytic enzymes and shows non-parallel changes to those of ATP citrate lyase. The amount of fructose-1,6-diphosphatase is also increased, suggesting an increased capacity for gluconeogenesis. These changes in enzyme amounts following allatectomy are discussed in relation to the function of the corpus allatum (CA) in adult development.