1. Protein synthesis was measured in muscle of fed and fasted rats. Synthesis rates were 29% higher with feeding. Treatment with indomethacin before feeding completely blocked the stimulation of protein synthesis. 2. Whole-body protein synthesis was measured in six healthy volunteers with [ 15 N]glycine in fasted and fed states. Feeding increased synthesis rates by 50%. An oral dose of indomethacin 2 h before the measurement of protein synthesis had no effect in either the fasted or fed state.