15N-Nuclear Magnetic Resonance (NMR) was used to study nitrogen assimilation in apices of maize roots in vivo, perfused either with 15NO⊟3 or 15NH+4 for long periods (17 to 20 h). Tissue saps prepared from roots grown in solutions supplied with 15NH+4 were also analysed by 15N-NMR. Perfusion experiments showed that the 15N label from nitrate or ammonium is first incorporated into the amide group of glutamine and then into the α-amino groups, mainly of glutamate/glutamine and alanine. Resonances assigned to α-amino groups of valine and asparagines/aspartate, to the side chain group of asparagine and to γ-aminobutyric acid could also be detected in vivo. Higher rates of label incorporation were observed when 15NH+4 was the nitrogen source. Furthermore, intracellular accumulation of 15NH+4 was observed. The results presented here suggest that the glutamine synthetase/glutamate synthase cycle is operating in this system under the experimental conditions used. Spectra obtained from saps of roots grown in the presence of 15NH+4 showed an increase of ammonium along the root during elongation and revealed the presence of asparagine together with glutamine in the elongation zone. By comparing spectra recorded in vivo with those obtained with the sap of equivalent root material, the absence of signals due to amino acids derived from glutamate (alanine, γ-aminobutyric acid) is clear in the latter case.