1. Energy cost of whole-body protein synthesis was measured in vivo in chicks by comparing the changes in protein synthesis and heat production after the administration of cycloheximide, an inhibitor of protein synthesis. 2. Incorporation of phenylalanine into whole-body protein fraction was promptly inhibited after the intravenous injection of cycloheximide, and the effect was sustained for at least 3 hr. 3. Both whole-body protein synthesis and total heat production were significantly reduced by the cycloheximide administration. 4. The energy cost of whole-body protein synthesis was calculated to be 5.35 kJ per g protein synthesis, and hence on a molar basis 7.52 ATPs are required per peptide bond synthesis.