Carboxyl-labeled and methylene-labeled IAA-14C were applied to the tips of two-day-old horizontal pea roots var. Alaska, and the tissue was subsequently halved and counted. In every experiment the lower halves of the subapical 4 mm became more radioactive than the upper halves. With an experimental period not over two hours the upper and lower halves contained an average of 33 and 67 per cent of the total radioactivity in the tissue respectively. Decapitation of the roots at 0.5 mm or more (removal of the entire root cap) prevented the transverse distribution of the applied auxin almost entirely. If the roots were allowed to absorb caffeic acid or 2,4-dichlorophenol prior to the application of the IAA-14C, the difference between the amounts of IAA-l4C present on the two sides was greatly decreased. The lAA-oxidase activity of homogenates made from the lower halves of the apical 3 mm of horizontal roots was less than that of homogenates from the upper halves. Decapitation did not affect this phenomenon. Apparently the unequal distribution of applied auxin and the difference between the activities of the lAA-oxidase on the two sides are not connected.