Budgets for carbon and nitrogen in shoot, root, and nodules of garden pea (Pisum sativum L.) are drawn up for a 9-d interval in the life cycle, from data on nitrogen fixation, carbon accumulation in dry matter, respiratory output of plant organs, and organic solute exchange between shoot and nodulated root. Of the carbon gained photosynthetically by the shoot from the atmosphere 26 per cent is incorporated directly into its dry matter, 32 per cent translocated to the nodules, and 42 per cent to the supporting root. Of the nodules’ share, 5 per cent is consumed in growth, 12 per cent in respiration, and 15 per cent returned to the shoot via the xylem, as amino compounds generated in nitrogen fixation. Growth and respiration of the root utilize, respectively, 7 and 35 per cent. The respiratory efficiency of a nodulated root in terms of nitrogen fixation (5.9mg C per mg N2-N fixed) is found to be very similar to that of an uninoculated root assimilating nitrate (6.2 mg C per mg NO3-N reduced). The nodules require in growth, respiration, and export 4.1 mg C (≡ 10.3 mg carbohydrate) for each mg N which they fix. The nodules consume 3 ml O2 for every 1 ml N2 utilized in fixation. In exporting a milligram of fixed nitrogen the nodules require at least 0.35 ml of water. Almost half of this requirement might be met by mass flow into the nodules via the phloem.