For studying the nitrogen metabolism in plants15N NMR spectroscopy can be used. Forin vivo15N NMR (natural abundance of15N: 0.37%) enrichment of the sample with the isotope15N is compulsory. The detection of time courses of15N assimilation from cells, which are enriched in culture is restricted in scope. Here, a method, the15N pulse labelling NMR spectroscopy, is demonstrated, which permits labelling of different nitrogen compounds in photoautotrophic microorganisms during the NMR spectroscopic measurement. Using an effective illumination system it is possible to maintain photosynthesis in plant samples of high biomass densities in the magnet necessary for ammonia assimilation. The technique thus enables to directly observe ammonia assimilation pathways by application of a15NO3−or15NH4−pulses.