[en] Techniques are described for studying the economy of carbon and nitrogen in annual nodulated legumes. Budgets for utilization of net photosynthate are constructed for cowpea (Vigna unguiculata (L) Walp.) and white lupin (Lupinus albus L.), including expenditure in respiration and dry matter accumulation of plant parts, carbon consumption in growth, respiration and export of fixed nitrogen by nodules, and the provision of recent photosynthate and earlier-fixed carbon to fruits. Sources of nitrogen to fruits are defined, and efficiencies of conversion of net photosynthate to protein of above-ground vegetative parts and of seeds are computed. Consideration is given to the timing of events associated with loss of symbiotic activity after flowering. Literature giving estimates of the respiratory requirements of nitrogen fixation by nodules is reviewed. Rates of respiration of nodules of cowpea, white lupin and pea (Pisum sativum L.) are assessed from a theoretical viewpoint, basing the estimates on ATP requirements for assimilation of N₂ into nitrogenous solutes, and published values for respiration costs in plant tissues. Expressed as CO₂ output per unit of nitrogen assimilated, these estimates greatly exceed the experimentally-observed CO₂ efflux of nodules of the species. This discrepancy is examined in relation to the capacity of nodules to fix CO₂ and the uncertainty of the in vivo requirement of nitrogenase for ATP