We tested four hypotheses for the control of partitioning of photoassimilated C-11 between the two halves of split root systems of young barley plants. Our data supported the hypothesis that phloem is unloaded without the use of metabolic energy, since several metabolic inhibitors applied to one half of a split root system reduced respiratory oxygen uptake without altering import of C-11. The hypothesis that rate of import C-11 is directly related to metabolic activity in the root was rejected, since (a) certain inhibitors reduced respiration but not import and (b) exogenous sucrose reduced import into the root half to which it was supplied. Our data were consistent with the hypothesis that import is related to the total ability (metabolism plus storage) of the sink to use sucrose. Treatments that would have led to greatly decreased use of sucrose (iodoacetate inhibition) decreased import before those which would have led to a smaller decrease in sucrose use (FCCP inhibition). These data, and the reduction in import to a root half supplied exogenously with sucrose, supported the hypothesis that the size of soluble sugar pools within the roots is, in the short-term, inversely proportional to rate of import, the soluble sugar pools thus acting as a mediator between rate of sucrose use and supply from the phloem.