Effects of increasing sink-source ratio on rate of translocation and net carbon exchange were studied by darkening all but one source leaf of Beta vulgaris L. or one primary leaf of Phaseolus vulgaris L. Rates of export of labeled material and patterns of its distribution among sinks were studied by means of GM detectors. Changes in export and import rates were compared with adjustments in starch, sucrose, and glucose levels in sugar beet source leaves before and during treatment. Sugar beet source leaf treatments which increased sink-source ratio had no sustained effect on rates of net carbon exchange, export, accumulation of starch, or the levels of sucrose and glucose on the day of treatment. However, the rate of import into a developing sugar beet leaf increased. Similar treatment of primary leaves of bean either had no effect on the rate of export of labeled material or caused up to a 20% increase. The increase in export was sufficient to account for the increase in import of labeled material into the far-sink leaflet. Nevertheless, distribution of labeled material exported to the sink regions changed; import of labeled material into the near-sink leaflet remained nearly unchanged and that into the roots Dec.reased. The data indicate that rapid changes in sink-source ratio cause a redistribution of the products of photosynthesis but need not involve a change in the rate of export.