It has been demonstrated by Pagan et al. (1998) that horses on a regular extensive exercise regime have an increased whole-tract passage rate and hence a decreased feed degradability. Recent work by Dougal et al. (2005) reported that regularly exercised horses had a decreased rate of gas production from faecal slurries compared to non-exercised horses, despite an apparently enhanced bacterial population. Many horses are not exercised during the week but are exercised at the weekend and receive supplementary feed after exercise. This may be fed in vain due to a decrease in feed degradability associated with increased exercise (Pagan et al. 1998). The aim of this experiment was to asses the impact of sudden short term exercise on the extent of microbial fermentation within the hind gut of the equine.