The carbon (C) and nitrogen (N) nutrition of cowpea cv. TVu 1503 plants nodulated with, and totally dependent on, one of two strains of Rhizobium known to produce different host plant dry weights and seed yields were compared with non-nodulated plants supplied with abundant nitrate-nitrogen. The development of both symbiotic associations was very similar until late in the reproductive period but thereafter nodules from strain R 5028 maintained larger relative fixation activity than those from strain CB 1024. Nitrate-dependent plants assimilated inorganic N more rapidly and produced more dry matter and larger seed yields than nodule-dependent plants. Remobilization of reduced N, which started sooner in non-nodulated than in nodulated plants, contributed between 23 and 31 per cent of total seeds. Current assimilation during late podfill was a more important source of N for seeds in nodule-dependent plants than those which relied on nitrate. Averaged throughout crop life, roots nodulated with strain R 5028 consumed 8·0 mg (estimated as root CO2 production) per mg N fixed and were 13 per cent more efficient than those inoculated with strain CB 1024. Plants dependent on nitrate were much more efficient during the vegetative period than those relying on nodule and, although they became very inefficient during fruit maturation, their overall root C consumption (as respired CO2) per milligramme N assimilated was only 56 per cent of the average for nodulated plants (4·5 mg). Respiration of nodulated roots utilized between 26 and 28 per cent of the C assimilated throughout growth compared with 18 percent when non-nodulated plants relied on nitrate. The contribution of studies on C-N relationships to yield improvement in grain legume crops is discussed.