Photosynthetic data collected from Pisum sativum L. and Phaseolus vulgaris L. plants at different stages of development were related to symbiotic N(2) fixation in the root nodules. The net carbon exchange rate of each leaf varied directly with carboxylation efficiency and inversely with the CO(2) compensation point. Net carbon exchange of the lowest leaves reputed to supply fixed carbon to root nodules declined in parallel with H(2) evolution from root nodules. The decrease in H(2) evolution also coincided with the onset of flowering but preceded the peak in N(2) fixation activity measured by acetylene-dependent ethylene production. A result of these changes was that the relative efficiency of N(2) fixation in peas increased to 0.7 from an initial value of 0.4. The data reveal that attempts to identify photosynthetic contributions of leaves to root nodules will require careful timing and suggest that the relative efficiency of N(2) fixation may be influenced by source-sink relationships.