The natural abundance of 15N in animal proteins (δ15Nanimal) is greater than that in the diet consumed by the animals (δ15Ndiet), with a discrimination factor (Δ15N = δ15Nanimal- δ15Ndiet) that is known to vary according to nutritional conditions. The objectives of the present study were to test the hypothesis that Δ15N variations depend on the efficiency of nitrogen utilisation (ENU) in growing beef cattle, and to identify some of the physiological mechanisms responsible for this N isotopic fractionation in ruminants. Thus, we performed the regression of the Δ15N of plasma proteins obtained from thirty-five finishing beef cattle fed standard and non-conventional diets against different feed efficiency indices, including ENU. We also performed the regression of the Δ15N of different ruminant N pools (plasma and milk proteins, urine and faeces) against different splanchnic N fluxes obtained from multi-catheterised lactating dairy cows. The Δ15N of plasma proteins was negatively correlated with feed efficiency indices in beef cattle, especially ENU (body protein gain/N intake) and efficiency of metabolisable protein (MP) utilisation (body protein gain/MP intake). Although Δ15N obtained from different N pools in dairy cows were all negatively correlated with ENU, the highest correlation was found when Δ15N was calculated from plasma proteins. Δ15N showed no correlation with urea-N recycling or rumen NH3 absorption, but exhibited a strong correlation with liver urea synthesis and splanchnic amino acid metabolism, which points to a dominant role of splanchnic tissues in the present N isotopic fractionation study.