Fractional protein synthesis rate was measured in the liver, and skeletal and cardiac muscles of newborn lambs, 2–5 days of, age. The contant infusion method was used and 14C-labelled leucine was given intravenously for 5 h. Blood samples were taken each hour and analysed for plasma leucine and specific activity; at the end of the experiment free (Si) and protein-bound (SB) leucine specific activity was measured in each organ. The calculated results indicated that more than 100% of the liver proteins were synthetised in a day in the newborn lamb, and that the synthesis rate of cardiac and skeletal muscle proteins were 35 and 25% day-1, respectively. Total leucine flux was 900 μmol/kg/day.