A split-root growth system was used to study photosynthate partitioning to developing nodules and roots of soybean (Glycine max L., Merr.). Opposite sides of the root systems were inoculated with Bradyrhizobium japonicum at 8 and 12 d after planting (early/delayed inoculation treatment) or, alternatively, only one side was inoculated 8 d after planting (early/uninoculated treatment). Plants were incubated with 14CO2 at 24-h intervals from early inoculation until the onset of N2 fixation (acetylene reduction). After staining with Eriochrome black, root and nodule meristematic structures were excised under a dissecting microscope and their radioactivity determined by scintillation counting. The specific radioactivity of nodule structures increased with nodule development, and was as much as 4 times higher in early nodules than in roots and nodules on half-roots receiving delayed inoculation. By the time that N2 fixation could be measured in the first mature nodules, the early inoculated half-root contained over 70% of the radioactivity recovered from the entire root systems of both early/delayed and early/uninoculated treatments. These results suggest that developing nodules create a strong sink for photosynthate, and that nodules and roots compete for current photosynthate. Early initiated nodules might develop at the expense of late initiated nodules, as well as at the expense of the roots themselves.