1. The rates of entry of twenty of the blood amino acids into skeletal muscle of living rats were measured directly by means of a technique which ensured that a steady concentration of a radioactively labelled amino acid is reached rapidly and is maintained in the bloodstream.2. The rates of entry were measured in experiments of short duration to avoid possible artifacts caused by amino acids leaving the muscle or by their metabolism.3. The entry rate of each amino acid increased in direct proportion to its concentration in the blood plasma over the physiological range.4. The various amino acids had widely different rates of entry. These rates could not be correlated with the physicochemical properties of the amino acids.5. Two amino acids, L-lysine and L-threonine, enter muscle against a concentration gradient, while in the case of a third, L-arginine, the blood concentration was raised high enough to induce saturation of the entry mechanism.6. It is concluded that entry takes place in vivo by means of carrier-mediated transport processes with a high degree of specificity.7. When the concentration of an amino acid in the bloodstream was increased to about twice normal the proportion of the additional amino acid that was taken up rapidly by the muscle was large enough, especially for the essential amino acids, to suggest that the tissue constitutes a quantitatively important storage system helping to regulate the concentrations of amino acids in the bloodstream.