The uptake of radioactive l -phenylalanine and l -tyrosine into the tissue water of rat brain cerebral cortex slices was shown to have saturation kinetics. The apparent K m for the uptake of l -phenylalanine was 0.86m m and for l -tyrosine was 1.64m m ; for phenylalanine the apparent V max. value was 0.64μmol/min per ml of tissue water, and for l -tyrosine it was 0.98μmol/min per ml of tissue water. The accumulation of the two amino acids by the tissue was depressed in the absence of O 2 , at 0°C, or in the presence of metabolic inhibitors. The influxes and effluxes of both l -isomers were more rapid than those of the d -isomers. Competition between these two amino acids and each with l -tryptophan in respect of uptake into tissue water was shown. Their rates of influx were faster, and rates of efflux were slower, in the presence than in the absence of Na + . It was concluded that these amino acids were taken up by active transport via a carrier mechanism.