1. Measurements have been made of whole-body and skeletal muscle protein synthesis in fed and fasted adults with l-[1-13C]leucine.
2. The marked increase in whole-body synthesis on feeding largely reflects the changes in protein synthesis in muscle, which doubles on feeding, compared with a 40% increase in that of the rest of the body.
3. Skeletal muscle in fed man contributes more than half to total protein synthesis occurring in the whole body.