Much controversy exists about whether or not NH+4 is translocated in the xylem from roots to shoots. In this paper it is shown that such translocation can indeed take place, but that interference from other metabolites such as amino acids and amines may give rise to large uncertainties about the magnitude of xylem NH+4 concentrations. Elimination of interference requires sample stabilization by, for instance, formic acid or methanol. Subsequent quantification of NH+4 should be done by the OPA-fluorometric method at neutral pH with 2-mercaptoethanol as the reducing agent since this method is sensitive and reliable. Colorimetric methods based on the Berthelot reaction should never be used, as they are prone to give erroneous results. Significant concentrations of NH+4, exceeding 1 mM, were measured in both xylem sap and leaf apoplastic solution of oilseed rape and tomato plants growing with NO-3 as the sole N source. When NO-3 was replaced by NH+4, xylem sap NH+4 concentrations increased with increasing external concentrations and with time of exposure to NH+4. Up to 11% of the translocated N was constituted by NH+4. Glutamine synthetase (GS) incorporates NH+4 into glutamine, but root GS activity and expression were repressed when high levels of NH+4 were supplied. Ammonium concentrations measured in xylem sap sampled just above the stem base were highly correlated with NH+4 concentrations in apoplastic solution from the leaves. Young leaves tended to have higher apoplastic NH+4 concentrations than older non-senescing leaves. The flux of NH+4 (concentration multiplied by transpirational water flow) increased with temperature despite a decline in xylem NH+4 concentration. Retrieval of leaf apoplastic NH+4 involves both high and low affinity transporters in the plasma membrane of mesophyll cells. Current knowledge about these transporters and their regulation is discussed.