1. Protein synthesis in mullet was measured by use of pulse injections of 14Cleucine into the hepatic portal circulation and by 4-hr continuous infusion of 14Ctyrosine into the dorsal aorta. 2. Liver protein synthesis was 37 mg/day per g wet weight of tissue. Plasma proteins accounted for about 25% of total liver protein synthesis. Replacement rate for intrahepatic proteins was 20%/day. Average polypeptide chain synthesis time was 3 min. 3. Gill protein synthesis was comparable to that in liver with a replacement rate of 23%/day. 4. Muscle protein synthetic rate was 0.54%/day, comparable to rates in rabbit and sheep when temperature difference is taken into account. 5. Constant infusion is shown to be a feasible technique for measurement of protein metabolism of fish in vivo.
