1. The fractional rate of protein synthesis (FSR) in tissues of nine growing lambs (4-5 months of age) was estimated following continuous infusion of L-[4,5-3H]leucine for a period of 7 h. Minimum and upper estimates of FSR were obtained assuming that the specific radioactivity (SRA) of leucine in blood plasma and tissue homogenate respectively defined that of leucyl tRNA. 2. Mean upper estimates of tissue protein FSR (/d) were skin 0.35, longissimus dorsi muscle 0.05 biceps femoris muscle 0.04, liver 0.54, rumen 0.79, cardiac muscle 0.09. Minimum estimates of tissue protein FSR ranged from 0.03 (muscle) to 0.15 (liver). 3. Plasma leucine flux was closely related to body protein content and dietary leucine absorption (r 0.94). 4. The rate of whole-body protein synthesis (WBS) derived from plasma leucine flux corrected for oxidation and localized recycling of leucine into protein was similar to that calculated from the sum of daily protein synthesis in individual tissues using the upper estimate of FSR, i.e. 610 g/d v. 581 g/d. 5. The estimate of WBS derived from plasma leucine flux directly (241 g/d) was similar to that calculated from the sum of minimum estimates of daily protein synthesis in individual tissues (214 g/d). 6. The ratio, intracellular leucine SRA:plasma leucine SRA tended to increase with increasing dietary leucine absorption in all tissues although these factors were only significantly correlated (P less than 0.05) in cardiac muscle, skin and rumen. Such relationships suggest an increased exchange of plasma leucine with intracellular leucine with increased food intake. 7. It was estimated that the energy cost of protein synthesis accounted for approximately 42% of daily heat production.