Nitrogen content in plant biomass yield of subterranean clover (Trifolium subterraneum L.) and cocksfoot (Dactylis glomerata L.) was determined as related to the different concentrations of nitrogen supply. Plants were grown sole and in mixture as symbiotrophic or heterotrophic cultures under controlled mineral elements concentration in the media and inoculation with rhyzobial strain. Two concentrations of inorganic nitrogen were tested: 0.125 mM (N1) and 1.25 mM (N2). Experimental variants: subterranean clover (100%) + N1; subterranean clover (100%) + N2; cocksfoot (100%) + N1; cocksfoot (100%) + N2; subterranean clover + cocksfoot (50:50%) + N1; subterranean clover + cocksfoot (50:50%) + N2. Inorganic nitrogen concentration had greater effect on shoot yield nitrogen content than on root yield nitrogen content for both crops. Nitrogen content in whole plant biomass yield of subterranean clover grown either sole or in mixture with cocksfoot was almost three times greater under high nitrogen concentration. Nitrogen use efficiency was highest in the mixture.