1. 1. The content of glycogen and free sugars DNA, RNA, protein and free amino acids extractable into 0.25 M sucrose from pupae of different ages has been studied. 2. 2. During the first 5 days of pupal life glycogen and protein were formed at the expense of free amino acids. 3. 3. In the period day 5 to day 20, the content of protein fell, while there was little change in glycogen plus sugars or in amino acids. 4. 4. Finally, when the developing exoskeleton of the fly was apparent, the protein content ceased to fall, and the amino acid level continued constant, while the glycogen and free sugars, DNA and RNA were depleted.