A series of experiments was undertaken to explored the general hypothesisin that the rate of production and accumulation of lactic acid is more important than the buffering capacity within the gut in the development of fermentative acidosis. A comparative study of the absorption of lactic acid, volatile fatty acids (VFAs), glucose and ammonia from caecal and rumen pouches was undertaken in anaesthetised sheep. Test solutions varying in pH, osmolarity, lactic acid concentration, and with fixed concentrations of VFAs, ammonia and glucose (100 mmol/L, 7 mmol/L and 4 mmol/L, respectively) were introduced into clean, surgically sealed pouches. Studies were undertaken in nine sheep, each with two pouches in the caecum and one in the rumen. Samples were taken at 10-minute intervals for 50 minutes to determine rates of absorption. On the other hand, the buffering capacities of caecal and rumen digesta of sheep fed oaten chaff and pasture and fasted were determined by titration with lactic, acetic and hydrochloric acids. On the basis of integrating the experimental data in this study and incorporating this with information already available in the literature, a computer modelling approach was developed using the program STELLA II to study acid production and absorption, and to predict pH during fermentation. ... The results are very useful for investigating effective methods and their mechanisms for the prevention and treatment of lactic acidosis. It appears that the rate of fermentation and the amount of substrate are important factors in terms of the development of fermentative acidosis.