There is controversy in the literature concerning the effect of amino acid (AA) and peptide nitrogen on microbial activity in ruminants fed diets high in structural carbohydrates. Microbial protein is the most variable and uncertain element of current systems of evaluating protein requirements for ruminants (ARC, 1984). Some of the variability is attributed to the multiplicity of techniques for estimating microbial yield, some of which are both imprecise and cumbersome. This study investigates the effects of different forms of nitrogen on microbial activity in sheep fed a rapidly degraded fibrous basal diet and attempts to validate the recently developed purine derivative (PD) technique by comparing it to other procedures used to estimate microbial protein synthesis under the same conditions.