The exchange of HTO with aerial portions of the soybean was studied under a variety of conditions. When fed to a lone leaf or leaflet in a saturated air atmosphere (all other leaves and the growing point having been excised), the HTO profile virtually ceases at a distance of 2 cm from the feeding chamber in the photosynthetic plant, but is greater and more extensive in the unilluminated plant. The differences are accentuated when roots are excised. Under these latter conditions the photosynthetic T-fixed gradient virtually disappears.HTO was exchanged with darkened petioles. When the rest of the shoot was kept in the light (the leaf being in a saturated H(2)O-vapor atmosphere) atmosphere) almost half the activity moves acropetally, and under these conditions (35 min, room temp) over 8% may be found in the leaf. Approximately one-tenth moves basipetally, with none being found in the stem. When the leaf is dark, no movement occurs out of the petiolar feeding chamber.An attempt was made to distinguish between sucrose transport by diffusion and mass flow of water by means of 2 mathematical models. In Model I, self-diffusion of HTO, Fick's Law was used, with the water and photosynthate moving independently. Model I consisted of an equilibrated, single pool of constant specific activity, generating a radioactive profile as a result of self-diffusion. In Model II, mass flow, water exchanged freely between the phloem and the surrounding tissues. The conducting bundle, 7790 micron(2), (0.25% of the total cross-section) was an average phloem. The numerical solution for the second model was obtained by Fortran programming on a digital computer and compared with experimental data. Comparison of these models with the experimental results suggest that mass flow is not a dominant process in soybean photosynthate translocation.