The relationshp between the relative abundance of ureides ([ureide-N/ureide-N plus nitrate-N] .times. 100) in the shoot axis (stems plus petioles), nodulated roots and leaflets of soybean (G. max [L.] Merrill cv. Bragg) and the symbiotic dependence of these plants was examined under glasshouse conditions. Plants, inoculated with effective Rhizobium japonicum CB1809, were grown with their roots exposed continuously to a nutrient solution containing either 0, 1.5, 3.0, 6.0 or 12.0 mM NO3-N/l. Nodulation and N2-acetylene fixation were correlated inversely with the level of nitrate. Seasonal acetylene reduction profiles for each of the nitrate treatments were integrated and the symbiotic dependence ([N2 fixed per total plant N] .times. 100) determined using a conversion ratio of 1.5:1 (acetylene reduced:N2 fixed), calculated from the zero NO3 treatment. Examination of the nitrogenous solutes of the shoot axis and nodulated roots showed linear relationships between the relative abundance of ureides and the symbiotic dependence of the plants. Two standard curves, depicting these relationships during vegetative and reproductive growth, were drawn for each plant part. The overriding effect of plant age invalidated any attempt to develop a standard relationship for leaflets. Data from 2 diurnal studies suggested that relative ureides were insensitive to diurnal fluctuations, thus simplifying sampling procedures. Plant material could be stored at ambient temperatures (20-30.degree. C) for up to 24 h without affecting the relative concentration of ureides and nitrate. Evidently, the shoot axis provides the most suitable target organ when using this technique as a quantitative assay for N2 fixation because of ease of sampling of these tissues, especially with field-grown plants.