The effects of Rhizobium inoculation and exogenous N concentrations [0y 0.5, 1.0, 2.0, 4.0 and 8.0 mM N as Ca(NO3)2] on plant growth, nodulation, N2 fixation, amino acid composition and ureide abundance in xylem exudates of nodulated cowpea (Vigna unguiculata L. Walp cv. Kausband) were investigated in glasshouse-grown plants in sand culture. Without applied N, inoculation markedly increased N2 fixation, top growth and allantoin concentration in exudates compared with the uninoculated control. Exogenous N, applied only to nodulated plants, had no beneficial effects on top, root or whole plant growth compared with inoculated control, but nodule mass and size and N2 fixation decreased substantially with increasing N concentrations exceeding 1.0 mM. However, nodule number was unaffected by any N concentration. Allantoin concentration and relative ureide index [(ureide N/ total N in exudates) x 100] declined drastically with N concentration, but the abundance of NO3N in the exudates followed the reverse trend. In plants fully dependent on Rhizobium symbiosis, allantoin was the predominant (94 %) export product recovered in xylem exudates, with the remaining N solutes comprising of NO3N (4.5%) and amino-N (1.5%). Conversely, NO3N was the principal (87%) form of N exported in xylem of NO3-dependent plants. Although 20 different amino acids were identified in the exudates, glutamine was the most abundant component (65 %) in the nodule-dependent crop whereas asparagine was the predominant (43 %) amino acid in the NO3 dependent plants. Nodule mass and relative ureide index were highly and positively correlated (r = 0.959**). Alkintoin concentration and N2 fixation had low correlation coefficient (r = 0.655**). The significance of these in biological nitrogen fixation studies is discussed.