Since the pioneer work of the Cambridge group into the physiology and biochemistry of ruminant digestion (Barcroft, McAnally and Phillipson, 1944), research has firmly established that the processes of digestion, synthesis and passage of nutrients within, and absorption of the end products from, the ruminant alimentary tract are extremely complex, highly interrelated and considerably influenced by the nature and quantity of the diet consumed (Baldwin and Koong, 1980). Consequently, the nutrients absorbed from the digestive tract have only limited quantitative and qualitative similarity to those consumed, mainly due to the processes of anaerobic fermentation and microbial synthesis occurring within the reticulo-rumen. The development of techniques to measure nutrient flow and microbial synthesis within the ruminant alimentary tract (Beever, Harrison, Thomson, Cammell and Osbourn, 1974; MacRae and Armstrong, 1969a; Smith and McAllan, 1970), has led to numerous studies on nutrient digestion and flow, mainly with sheep fed forage and cereal based diets (Beever, Thomson and Cammell, 1976; Beever, Thomson, Cammell and Harrison, 1977; Beever, Terry, Cammell and Wallace, 1978; Coehlo da Silva, Seeley, Beever, Prescott and Armstrong, 1972; Coehlo da Silva, Seeley, Thomson, Beever and Armstrong, 1972; Egan, Walker, Nader and Ulyatt, 1975; Faichney and Weston, 1971; Faichney and White, 1977; Harrison, Beever, Thomson and Osbourn, 1973, 1975; Hogan and Weston, 1970; MacRae and Armstrong, 1969b; MacRae and Ulyatt, 1974; Siddons, Evans and Beever, 1979; Ulyatt and MacRae, 1974) and, from some of these studies, empirical relationships have been established (Beever et al., 1976; Chalupa. 1975; Ulyatt and Egan, 1979; Weston and Hogan, 1973) in an attempt to predict, from a knowledge of feed input and composition, the likely end products of digestion. However, considering the complexities of ruminant digestion, the limited applicability of such relationships was not surprising and the need for a more fundamental consideration and integration of the processes of digestion and synthesis was established. To this end, the work of Baldwin and his colleagues (Baldwin, Lucas and Cabrera, 1970; Baldwin, Koong and Ulyatt, 1977; Reichl and Baldwin, 1975) stimulated interest in the area of rumen modelling.