A lot of controversy in literature still surrounds the way in which free amino acids (AA), peptides and true protein nitrogen influence fibre digestion and the activity of bacteria which ferment structural carbohydrates in the rumen. One of the major impediments in the understanding of ruminal microbial protein synthesis is the lack of a simple and accurate procedure for measuring its yield. The aim of this study was two fold: firstly, to make further investigations into the effects of feeding different forms of supplementary nitrogen to a rapidly fermented high fibre diet on microbial activity, and secondly, to compare three different procedures of estimating microbial protein synthesis under the same conditions in continuous culture (CC).