An increasing sugar gradient in the rib, a decreasing gradient in the upper region of the petiole, and a sharp increase in the base of the petiole were found when transverse segments of the rib and petiole of a sugar beet leaf or the isolated vascular bundles as a whole were analyzed. The sugar concentration and the gradient pattern varied considerably with the leaf position in the ontogenic sequence. The proportion of assimilated14C exported as sucrose was highest for leaves of intermediate age, which were at the nearly mature stage. Mature and old leaves exported less14C and young leaves, which had a high sugar concentration in the blade and a sharply decreasing gradient in the petiole, exported very little. Analysis of vascular bundles in the petiole which serve the tip and basal regions of the blade after14CO2incorporation into the blade indicated that a much larger amount of14C-sucrose entered into and moved through the basal bundle than into and through the tip bundle.