Non-degradable dietetic protein supply depends on rumen degradation. Determination of microbial protein synthesized in the rumen as the result of microbial fermentation is important because microbial protein synthesis could be influenced by diet (Dove and Milne, 1994). Several evaluation systems consider the contribution of microbial protein on protein intestinal flow as a constant, based on feed intake. But this approach has presented great variations. The purpose of this work was to determine in vivo microbial protein kinetics for sheep fed with three protein level hays.