Ruminant animals acquire their amino acids from the small intestinal digestion of ruminally derived microbial protein and dietary protein which has escaped ruminal degradation. The UK metabolizable protein system provides a framework with which the net absorption of amino acids from the small intestine is computed in relation to the animal`s requirements and is based largely upon a set of criteria by which feedstuffs are evaluated. While conceptually the system has many positive features, a number of serious weaknesses have been identified, particularly with respect to the techniques used to evaluate feedstuffs. Some of these issues are considered, particularly the over-reliance which is placed upon in vitro methodologies which have not been adequately validated against in vivo observations and have been shown to give unacceptable variation. Attention is also drawn to the inadequate representation of host tissue metabolism within such systems, for example, the importance of splanchnic metabolism with respect to overall nutrient utilization. It is concluded that improved representation of energy (i.e. carbohydrate) and protein interactions within the whole animal and not simply within the gastro-intestinal tract is urgently required; this inevitably will lead to a change in research direction. Equally, support for a mechanistic understanding rather than an empirical representation of protein metabolism in ruminant livestock is presented, given that the demands upon the ruminant industry, particularly with respect to predictability of animal response, is likely to increase as a consequence of increased consumer impact on the marketing of animal products.