The determination or estimative of microbial protein supply is important to study the protein nutrition for ruminants. Microbial protein contribution to the total gut protein flow is very often considered as a constant based on feed intake. There are a great number of factors that can interfere on microbial synthesis and they are very difficult to be studied all together. The use of the in vitro radiophosphorous (32P) incorporation technique allows studying isolated factors, because it is possible to control some of the factors involved. The aim of this work was to evaluate microbial nitrogen yield from poor, moderate and good quality hays and its influence on quality of rumen microbial population of sheep fed these hays.