Eight confined lactating cows at mid lactation fed diets based on silages were used in an incomplete Latin square design to evaluate the effects of increasing rumen undegradable protein (RUP) on nitrogen utilization and excretion. Four diets were used. Diets designed as AI and B I had, on a DM basis, 45% pasture silage and 20% alfalfa haylage, while diets A2 and B2 had 38% permanent pasture and 27% alfalfa silage. Diets A and B differed mainly on the level of RUP. Diets A were formulated accepting up to a 20% RUP deficit and included soybean meal, while B diets covered 100% of estimated RUP requirements, using fish meal as a protein source. In spite of causing a lower N intake (P < 0.05), diets higher in RUP (B diets) resulted in similar milk production. Productive N (secreted as milk nitrogen plus N in daily gain, expressed as g day(-1) and as g kg(-1) of N intake) was improved significantly (P < 0.05) in higher RUP diets. Faecal and urinary N excretion were also lower (P < 0.05 and P < 0.01, respectively) in these diets. Intake N used in productive functions was higher in B diets (P < 0.01). The relationship productive N v/s intake N and excreted N (faeces plus urine) per litre of milk produced, and productive N v/s excreted N were all improved with B diets. These data indicate that correcting a RUP deficit of high silage diets with fish meal, increased the efficiency of the use of N for productive functions and decreased potential contamination through a reduced N excretion.