The effect of N nutrition on root xylem (water) flow in Phaseolus vulgaris was studied by using the 1H NMR flow imaging technique. Plants pre-cultivated on CaSO4-solution were transferred to NMR-compatible split-root cuvettes filled with half-strength nitrate medium (2.5 mM). After 6 days full-strength (5 mM) nitrate medium was supplied to both compartments (NN treatment), or only to one compartment, whereas the other one was perfused with full-strength (5 mM) ammonium medium (NA treatment). Imaging was performed 7–8 days later. Root growth, xylem flow velocity and xylem pressure (measured with the xylem pressure probe) were largely unaffected by N nutrition. Under NA conditions, volume flow was significantly lower in ammonium- compared to nitrate-fed roots due to a reduced number of functional xylem elements. This was apparently not related to anatomical differences nor to an enhanced number of cavitated vessels in ammonium-fed roots. Vessels in these roots may become dysfunctional by clogging to prevent cavitation.