Peptides and to a lesser extent amino acids accumulate in rumen fluid in the early post feeding period and rapidly decline thereafter (Broderick & Wallace, 1988). Numerous studies have demonstrated benefits to feeding peptides, in terms of increased microbial growth in the rumen (Newbold, 1999). However, given that peptides will only be available in the rumen for a short time after feeding it may be necessary to match supply of peptides and energy in the rumen to maximise the stimulation in microbial activity. The objective of this study was thus to investigate if microbial protein synthesis in rumen fluid would be enhanced by a synchronous provision of peptides and energy.