The translocation of 14C-assimilate in sugar beet can be stopped at the top of the petiole when cyanide (pH 7,0.5 M) is applied to the petiole about 12 cm from the base of the blade 3 min before the 14CO2 assimilation. Such an inhibition persists for at least 50 min. The possible retarding effect of cyanide on photosynthesis, the loading process of sugar into the venation, and movement of sucrose along the conduit have been investigated by chromatographic analyses of the photosynthetic products and by radioautography of the 14C movement in the leaf blade. The cyanide moves rapidly upward from the point of application in the petiole, but does not affect the photosynthetic or loading processes in the leaf blade. The results suggest that a cyanide-sensitive mechanism located in the phloem tissue along the conducting pathway is essential to maintain the normal rapid movement of assimilate from the source to the sink.