Microbial protein synthesis is essential to meet protein requirements in ruminants. The aim of this investigation was to evaluate the effect of incorporating extruded corn (Zea mays L.) into the ration of young steers fed with grass silage on microbial protein synthesis (MPS). Twenty young steers in metabolic cages were used for supplying food. The treatments consisted of combinations of extruded corn (EC) and ground corn (GC) EC-10 (10% EC-30% GC); EC-25 (25% EC-15% GC) and EC-40 (40% EC-0% GC). These foods constituted 40% of the ration (DM-based) and the remaining 60% was grass silage. The MPS was calculated through the excretion of purine derivatives (PD) in the urine, using total collection of urine. The inclusion of a higher percentage of extruded corn (EC-40) increased the PD excretion (51.85 mmol d-1, P < 0.05). The values of PD excretion ranged between 663 and 1078 &#956;mol BW-075 d-1. The microbial protein absorption and flow of duodenal N also increased with a higher portion of extruded corn (EC-40, P < 0.05). The differences between treatments can be explained by the effect of corn extruding. This allows more energy available for ruminal bacteria and improved synchrony between energy and protein.