1. Rat heart was perfused with a mixture of amino acids and other ninhydrin-positive substances in the same concentrations as they are found in plasma, and with or without sufficient puromycin to suppress protein synthesis all but completely. 2. In the presence of puromycin, but not in its absence, insulin increased the concentrations of most of the individual amino acids in the intracellular water of heart. 3. The results accord with the conclusion that insulin increases the entry of most, if not all, amino acids into muscle.
