Glutamine synthetase (GS) and glutamate dehydrogenase (GDH) are both present inSphagnumand in principle both enzymes could be involved in the assimilation of ammonium.15NH4* application led to the formation γ-15N-glutamine, followed by α-amino glutamate/glutamine as observed byin vivo15N NMR. No assimilation was detected in the presence of the GS-specific inhibitor methionine sulfoximine. This labelling pattern suggests, that ammonium assimilation inSphagnumis mediated solely by the GS/glutamate synthase pathway. The application of15N-labelled glutamate led to the liberation of15NH4*, indicating that GDH mainly catalyses the deamination of glutamate. It was possible to record good-qualityin vivo31P NMR spectra ofSphagnum flexuosumtissue, and the chemical shift of the orthophosphate resonances indicated that cytoplasmic pH was not affected during the ammonium application, whereas the vacuolar pH was slightly reduced.