Mammary cells from lactating rats and dairy cows were cultured in Eagle's minimal essential medium (MEM) with added amino acids. Changes in free intracellular amino acid pools and milk protein synthesis in response to amino acid additions to the medium were measured. Increases in free intracellular amino acid pools are associated with increased protein synthesis and the rate of their change in response to extracellular amino acids would partially reflect the cell's amino acid requirement. The intracellular pools from medium amino acids (except methionine, tryptophan and glutamine) increased with extracellular amino acids but at their own characteristic rate. The ratio of medium amino acids to nonmedium amino acids inside the mammary cell increased with the concentration of amino acids in the medium. Methionine and tryptophan did not have measurable pools in the rat mammary cell and only very small pools in the bovine mammary cell which did not increase with extracellular amino acids. Culturing rat mammary cells with labeled methionine showed only 42% of the intracellular radioactivity was still associated with labeled methionine indicating significant conversion of this amino acid after it entered the cell. A small linear increase in intracellular cystine was observed with elevated cystine in the medium. The responses in cystine, tryptophan and methionine intracellular pools to extracellular amino acids suggest the concentration of these amino acids in the medium are insufficient to meet the bovine mammary cells requirement. Increasing the concentration of amino acids in MEM 1-, 3-, 5- and 7 fold significantly (P < .05) increased β-casein and to a lesser degree β-lactoglobulin synthesis by bovine mammary cells in culture. Individually increasing each of the 13 amino acids in MEM 3-fold showed cystine followed by threonine and then methionine significantly (P < .05) increased the combined synthesis of β-casein and β-lactoglobulin. The correlation between intracellular pool size change and the response in milk protein synthesis to increased individual amino acids was -.51 which was not significant at the .05 level. / Ph. D.