1. A new stem-puncture technique was developed to study xylem transport of regulating chemicals and other biologically active substances. A fine wire made into the shape of a chisel was inserted into a relatively immature section of bean stem so as to sever two strands of tracheary elements in the primary xylem. The regulating chemical, applied as an aqueous mixture, was sucked into the puncture and translocated up the severed elements into a leaf with which they were associated. 2. When C14-tagged a-methoxyphenylacetic acid was used, the compound in some instances translocated upward in detectable amounts at a rate equal to 60 feet an hour following introduction into the xylem. 3. When 10 μg. of the regulating chemical were applied by means of the puncture technique, approximately 2 μg. were translocated into the leaf associated with the severed tracheary elements within a period of about 5-6 minutes, following which little transport occurred. Twenty-six times as much of the regulator was translocated from the stem to the leaf following use of the puncture technique as was translocated over the same path when the chemical was applied and the stem was left uninjured. 4. Of the absorbed and translocated regulator, 58% accumulated in the leaf associated with the severed tracheary elements, and most of the remainder moved to other parts of the plant above the punctured area. Downward movement of the tagged chemical was essentially nil except after the compound reached the leaves and entered the phloemtransport system, where its downward movement became entirely light dependent. 5. Exudation of the regulator from the roots was light dependent apparently because of the inability of light-deficient plants to translocate the chemical downward from their leaves. Illumination of all leaves on a plant except the one containing the growth-regulating chemical failed to induce downward transport of the regulator from the one leaf kept in darkness.