The objective was to quantify the flow of intestinal nutrients and nitrogen excretion and retention in sheep receiving isoproteic diets. Eight Texel x Lacaune wethers (average body weight = 25±2.5 kg) were fitted with duodenal cannula and housed in metabolic cages. Wethers were assigned to the treatments in a crossover design with two periods of 20 days each, and all feces and urine produced by the wethers were collected. The treatments consisted of two isoproteic (160 g kg−1 of crude protein on dry matter basis) diets composed of red clover (RC) or lucerne (LU; Medicago sativa) silages plus corn silage and concentrate feed. The digestible organic matter and metabolizable energy intake did not differ between treatments. The intestinal non-ammonia N (NAN) flow was 5.9 g day−1 (37%) higher in RC wethers than in those of the LU treatment. This result was a consequence of both an increase in the efficiency of microbial protein synthesis (12.7% higher) and a decrease in ruminal degradable protein (RDP) content (20% lower) of the diet. However, the increase in the intestinal NAN flow was accompanied by a reduction in intestinal digestibility of N, resulting in similar daily N retention between treatments. The reduction of RDP content was probably the main reason for reductions in N urinary excretion in RC wethers compared with those in the LU treatment, showing that RC silage may be a tool for mitigating the impact of N excretion in ovine production systems, without changes in N retention.