ALTHOUGH protein synthesis is obviously dependent on availability of amino-acids1, the relationship of alterations in intracellular levels of amino-acids to rates of protein synthesis and breakdown remains obscure. Thus, enhanced protein synthesis has been observed in the presence of normal2 as well as elevated3 cellular levels of amino-acids. A maximum effective cellular concentration may exist for each amino-acid beyond which net protein synthesis cannot be further stimulated4,5. However, above this concentration alterations in gross protein synthesis may occur which are masked by concomitant variations in protein breakdown. The investigations recorded here were designed to investigate this possibility. The data indicate that a primary increase in extracellular concentration of one essential amino-acid may result in stimulation of protein synthesis and protein breakdown in surviving liver cells.