—An attempt was made to isolate the saturable uptake from the unidirectional influx of amino acids into tissue slices and to estimate the transport constants and maximal velocities of saturable transport. The method was applied to studies on the inhibition of phenylalanine in the saturable influx of tyrosine, tryptophan, histidine and leucine into brain cortex slices from adult and 7-day-old rats. In both age groups phenylalanine inhibited the influx of the other amino acids, and vice versa. The apparent transport constants of the other amino acids increased in the presence of phenylalanine more noticeably in the slices from 7-day-old rats than in those from adult rats, whereas the concomitant influx of phenylalanine was inhibited less in the slices from 7-day-old rats. In immature animals in vivo competition between amino acids may play a more marked role in the supply of amino acids from plasma to brain, as the transport systems in brain slices from 7-day-old rats become saturated with extracellular amino acids more readily than do the transport systems in brain slices from adult rats.