The consequences of chewing during eating and rumination of ingesta on the reduction of feed particles has been studied extensively in ruminants (Kennedy, 1984). These studies have lead to the development of the concept of critical size, that is the minimum reduction of feed particle size to allow passage from the rumen. However, feed particle size ingested seems to have little effect on reduction of particles to critical size as a result of chewing and rumination to facilitate passage from the rumen (Faichney, 1990). In the horse once feed has passed the cardiac sphincter into the stomach, no post ingestive mastication can occur. This may be an important component in the apparent reduction in the digestibility of energy substrates in the horse compared to the ruminant. The study reported here examines the relative particle size distributions of faeces of horses and sheep given identical diets.