Comparative rates of incorporation of sulphate and L-leucme into proteins have been determined from autoradiographs for the cells of root apices. This has confirmed the existence in most roots of a quiescent centre where the rate of synthesis is much lower than in the surrounding cells. A more detailed account of leucine incorporation in the roots of Zea has been made by counting silver grains in quadrats on the autoradiographs. The rate of incorporation per unit volume increases on either side of the quiescent centre and then falls. On the proximal side the lowest rate of incorporation is below that in the quiescent centre and is reached at 1–2 mm. from the tip. Farther from the tip the rate rises. The rate of incorporation per cell does not alter much in the first a mm. of the meristem, but beyond that the rate rises steeply to between 15- and 5o-fold at 3 mm. from the tip. The patterns of change in rates of incorporation are different for the different tissues of the root.