14C (U) L-glutamic acid was supplied to the roots of young tomato plants. After a four hours’ uptake period all the amino acids present in the root proteins were weakly labelled, but of the free amino acids of the roots only y-aminobutyric acid, glutamine, glutamic acid and aspartic acid proved to be active. In addition, the label was incorporated into lipoids and a number of carboxylic acids. The extent of labelling of the various leaves appeared to be correlated with the synthetic activities of these leaves, and inversely related with leaf age. The distribution was not influenced by the presence or absence of roots. It is concluded that the xylem is the initial way of translocation of amino acids, which are synthesized in the roots. During their movement through the xylem vessels to the areas of transpiration, the amino acids are partly absorbed by surrounding tissues and translocated to centres of protein synthesis.