Interactions between nitrogen and water may vary between N-2 fixing species and species that rely only mineral soil nitrogen. Here we compared above ground biomass accumulation (W), nitrogen uptake (N), evapo-transpiration (ET) of alfalfa (Medicago sativa L.) and tall fescue (Festuca arundinacea Schreb.) during regrowth periods under contrasting supply of water (irrigated vs non-irrigated) and N (for non-fixing species). Using previously published data, we estimated the dynamics of the two components of ET, evaporation from soil (E) and transpiration (T), in order to analyse the impact of E/T, of the transpiration efficiency (TE = W/T) and the role of crop N nutrition on water use efficiency (WUE = W/ET). In tall fescue, limiting N supply reduced WUE by both increasing E/T ratio and decreasing TE. Water limitation in both alfalfa and tall fescue led to crop nitrogen deficit. This drought-induced N deficiency resulted in a proportional reduction in TE irrespective of the source of N for the plant. We propose that the ratio N/T, representing the apparent N concentration of water transpired by the crop, is relevant for analysing N-water interactions. Comparisons of dynamics of N/T ratio must be done at similar biomass or similar transpiration because N and T are related allometrically.