This paper summarizes some recent advances in the understanding of nitrate assimilation in the model legume Lotus japonicus. First, different types of experimental evidence are presented that emphasize the importance of the root in the nitrate-reducing assimilatory processes in this plant. Secondly, the main results from an ethyl methanesulphonate mutagenesis programme are presented. In this programme, chlorate-resistant and photorespiratory mutants were produced and characterized. The phenotype of one particular chlorate-resistant mutant suggested the importance of a low-affinity nitrate transport system for growth of L. japonicus plants under nitrate nutrition. The phenotype of photorespiratory mutants, affected in all forms of plastid glutamine synthetase in leaves, roots, and nodules, indicated that plastid glutamine synthetase was not required for primary nitrate assimilation nor for the symbiotic associations of the plant (nodulation, mycorrhization), provided photorespiration was suppressed. However, the phenotype of these mutants confirmed that plastid glutamine synthetase was required for the reassimilation of ammonium released by photorespiration. Finally, different aspects of the relationship between nitrate assimilation and osmotic stress in L. japonicus are also discussed, with specific reference to the biosynthesis of proline as an osmolyte.