The sphincter of Oddi (SO) is a neuromuscular structure located at the junction of the bile and pancreatic ducts with the duodenum. The primary functions of the SO are to regulate the delivery of bile and pancreatic juice into the duodenum, and to prevent reflux of duodenal contents into the biliary and pancreatic systems. Neural, hormonal or functional disturbances of biliary motility can lead to painful and sometimes life threatening clinical conditions, such as SO dysfunction and acute pancreatitis. Clearly understanding the regulation of biliary and duodenal motility patterns is necessary and may provide useful pharmacological sites for drug development to aid in the treatment of these diseases. Spontaneous activity of the SO is regulated by complex interactions between the enteric nervous system, hormones, possibly interstitial cells of Cajal and other bioactive agents, together with modulation via neural reflexes between the duodenum, common bile duct/gallbladder, and stomach. Purines are one group of neurotransmitters/regulatory agents that have been shown to effect gastrointestinal motility, however their functions in the regulation of SO motility have not been elucidated. The studies described in this thesis used in vitro organ bath techniques and in vivo preparations to determine the effects of exogenous purines on possum SO and duodenal motility. The possum SO has been extensively characterized and is an excellent model for motility studies. In vitro, exogenous adenosine was found to decrease spontaneous activity in both the SO and duodenum. In contrast exogenous ATP induced both excitatory and inhibitory responses in the SO and duodenum. Interestingly, the adenosine and ATP-induced effects were predominantly exhibited by the proximal portion of the SO (proximal-SO), with no or little effect observed in the distal portion of the SO (distal-SO). These data support the hypothesis that the SO is comprised of different functional components that can act differently in response to certain stimuli, and highlights the importance of studying each of the SO components. Agonists and antagonists, together with immunohistochemical studies, were used in an attempt to identify the P1 and P2 receptor sub-types responsible for mediating the adenosine- and ATP-induced responses. In the duodenum the adenosine-induced decrease in spontaneous activity was likely to be mediated by A2A and A3 receptors, but the receptors mediating the proximal-SO response could not be identified. In the duodenum ATP induced a complex non-neural response consisting of a P2X1, and P2Y2 and/or P2Y4 mediated immediate inhibition. This was followed by a return to baseline activity or small excitation. The response concluded with a late inhibitory response, likely to be mediated by P2Y1 receptors, but the effects of other P2Y receptors could not be excluded. In contrast, ATP application to the proximal-SO evoked a partially neurally mediated early excitation, likely via P2X receptors, followed by an inhibition of activity, likely via activation of non-neural P2Y2 and/or P2Y4 receptors. In vivo studies with exogenous application of adenosine and ATP to the SO activated neural pathways to produce increased motor activity. Characterisation of these neural pathways found ATP and/or adenosine to activate excitatory cholinergic motor neurons. ATP also activated an inhibitory nicotinic/nitrergic pathway. This is the first comprehensive investigation of the possible involvement of purines in the regulation of SO motility. These studies demonstrate that exogenous purines influence SO and duodenal motility, inducing complex neural and non-neural responses, acting via multiple P1 and P2 receptors. It now remains to be determined if endogenously released purines induce similar responses, together with elucidation and location of the receptor sub-types involved.