This thesis describes investigations of small intestinal motility in horses with and without equine grass sickness using an in vitro technique. Equine grass sickness is a disease of horses characterised by dysfunction of the autonomic nervous system, primarily affecting the alimentary tract. Examination of autonomic ganglia taken from horses affected with grass sickness shows evidence of neuronal degeneration and ultimately depletion of cell numbers. The aetiology of grass sickness is still unknown. The in vitro technique used strips of intestinal smooth muscle cut parallel to the longitudinal muscle layer from the duodenum and ileum. Tissue was taken from control horses and those affected with the three clinical forms of grass sickness (acute [AGS], subacute [SAGS] and chronic grass sickness [CGS]). Motility patterns were measured isometrically using strain gauge transducers and recorded onto a Washington ink writing oscillograph. Contraction rate and amplitude, alterations in tone (baseline) and the latency before a response to pharmacological agents were recorded. The characteristics of the background contractions were established. In the control group the duodenal preparations had a significantly higher contractile rate than ileal preparations (P<0.05). There was no significant difference in contractile amplitude between the two regions. The contractile rate was reduced in grass sickness cases, although not always significantly. The effect of storage for 24 hours at 4^oC was investigated to see if stored control tissue would subsequently behave like fresh grass sickness tissue; it was concluded that this was not the case. Physostigmine was used to test the viability of enteric cholinergic neurones and their capacity to release endogenous acetylcholine. All muscle strips from both control horses and those affected with grass sickness showed significant increases in the rate of contractions following a physostigmine addition (P<0.05 or less). The latency before a response to physostigmine in the AGS and SAGS groups was significantly larger than for the control groups in both regions of the gut (P<0.001 for duodenal tissue, P<0.05 for ileal tissue).