Synaptosomes isolated from adult or newborn rat cerebrum take up L-lysine by two saturable systems, one with a high affinity low capacity and the other with a low affinity high capacity. Initial rate of uptake for low lysine concentrations is mort tissue. Analysis of kinetic data indicates that synaptosomes of the newborn have a higher Vmax than those of the adult for high affinity system but adult for high affinity system but adult synaptosomes have a higher Vmax than newborn for low affinity system. At a physiological lysine concentration of 0.5 mM, the calculated contributions of two systems indicate that the adult uptake occurs for about 71% by low affinity system but the newborn utilizes both systems to the same extent. The uptake is sodium independent but pH dependent. Lysine uptake is inhibited by other dibasic amino acids, arginine and ornithine but not cystine. Kinetic analysis indicates that arginine specifically inhibits the high affinity, low Km system for lysine uptake.