The products of CO2 and N2 fixation in alfalfa (Medicago sativa L.) were followed after supplying 14CO2 to the shoot system while the root was exposed to 15N2 or after supplying both labels to the root. Photosynthate was transported to the nodules in less than 1 h and was distributed throughout the plant primarily in the neutral (sugars) fraction (over 80%), while the basic (amino acids and amides) and acidic (organic acids) fractions each contained only 5 to 10% of the total radioactivity. At midday the nodules fixed more than twice the amount of carbon that they received as fresh photosynthate. Calculations showed that nodule dark CO2 fixation may contribute about 12% of the total carbon-skeleton requirement for the assimilation of fixed nitrogen. The assimilation of fixed nitrogen appears to use carbon derived from a pool of photosynthate that accumulates in the nodules in the morning. The products of nodule dark CO2 fixation, which amounted to about 2.5% of total plant CO2 fixation, were partitioned to the shoots after 30 min. Most of the radioactivity (70%) from nodule dark CO2 fixation was incorporated into the basic fraction; the rest (30%) was recovered in the acidic fraction and little radioactivity was detected in the neutral fraction. Fixed nitrogen from 15N2 was rapidly assimilated into organic compounds, at about 22 μmol · plant−1 · h−1, especially into Asn (80%), with only a small portion of the nitrogen remaining in the form of NH3. The fixed and assimilated 15N accumulated first in the nodules and then was rapidly transported to the shoots for further metabolism.