The adequacy of the concepts and of the quantitative information incorporated into a computer program which simulates rumen function in sheep has been assessed by observing the sensitivity of the predictions to changes in variables requiring better numerical description and by comparing the predictions with observations made for different dietary situations. For all diets, the predicted amount of total protein flowing from the rumen was highly sensitive to changes in the potential degradability of dietary protein, the fractional outflow rates from the rumen of water and of microbes, and the energy required for maintenance of the microbial population. For diets in which nitrogen limited microbial growth, it was also highly sensitive to changes in salivary flow, which represents the recycling of nitrogen to the rumen, and to the proportion of rumen ammonia available for microbial growth. For all diets, the predicted outflow of total protein from the rumen showed medium sensitivity to changes in the composition of microbial organic matter; for high fibre diets, it showed medium sensitivity to changes in the potential degradability of β-hexose; and for diets high in starch, it showed medium sensitivity to changes in the fractional outflow rate of α-hexase. For diets in which inorganic sulphur availability limited microbial growth, the model predictions showed medium sensitivity to the proportion of rumen inorganic sulphur available to the microbes. Because these variables could not be set with confidence, it is clear that research is needed for their better definition. For many dietary situations the pattern of the response in model predictions closely followed that of experimental observations. However, the inability to predict the effects of increasing rumen solute load, grinding and pelleting of forages or the recycling of endogenous nitrogen when microbial growth was limited by nitrogen availability, emphasises the need for research in these areas.