1. Biochemical and histochemical analyses were performed on skeletal rat muscles over the course of development from neonate to adult. 2. A small amount of triglyceride and a large amount of glycogen were found to be contained in neonatal type 2C muscle fiber; based on this finding the main energy metabolism in this fiber was thought to be anaerobic glycolysis. 3. Moreover at 10 days type 2C muscle fiber was shown to contain a large amount of triglyceride. 4. These results suggest that, although histochemical characteristics remain the same, biochemical properties change during the course of muscle development.