The first research on the transport of metastable-technetium-99 ( 99mTc) in the form of pertechnetate (TcO4-99m) within plants suggested that TcO4-99m may be mobile in the phloem. In contrast, more recent evidence indicates the anion is transported in the xylem. Here we demonstrate that observations of 99mTc transport in the test subject of these initial investigations, the moonflower (Ipomoea alba L.), are incompatible with phloem flow. Rather, the presence of only minute amounts of 99mTc in typical sinks for phloem solutes and 99mTc transport out of labeled leaves when shaded but not when illuminated strongly suggest that the radionuclide is transported in the xylem. The study increases confidence in the identification of TcO4-99m as a xylem mobile compound whose distribution in plants can be visualized using nuclear medicine scintigraphic imaging techniques.