A solution of 5 mM 14C-L-ot-alanine in tritiated water was allowed to perfuse the xylem vessels of tomato stem segments. In each stem segment an exponential distribution gradient for THO was found, the slope of which was dependent on the length of the stem segment and the perfusion velocity. Nearly all the THO was retained in the stem segment whereas a great amount of 14C-alanine could be recollected. Thus the 14C-alanine was recollected in a solution of other water molecules than those it was introduced in. A similar phenomenon could be observed, if 14C-L-a-alanine or 14C-D-sucrose dissolved in THO were allowed to flow through a Sephadex G-10 column. The results clearly demonstrate, that mass flow in xylem vessels goes along with exchange and diffusion-like processes. Moreover experimentally they show that different velocities for the movement of organic solutes and THO in other long distance translocation systems (sieve tubes) – as reported in literature – need in no way be in conflict with an assumed longitudinal mass flow of their contents.