1 The aim of this study was to determine if stimulation of duodenal motility by duodenal fluid distension or by administration of carbachol, activates the sphincter of Oddi-duodenal reflex, in an in vitro preparation from the Australian possum. 2 Duodenal distension was achieved by infusion of Krebs solution (0-8 cm H2O). In separate experiments, the sphincter of Oddi (SO) was partitioned from the duodenum and carbachol (10(-7) - 5 x 10(-6) M) added to the duodenal compartment. 3 Fluid distension increased duodenal motility to 120-600% of control activity. These treatments induced increased SO motility (to 120-390% of control) in six preparations, reduced activity (to 60% of control) in one and no response in another. 4 Addition of carbachol to the duodenal compartment resulted in increased duodenal motility. SO motility was increased in seven preparations, reduced in another two and no response were evoked in two others. All SO responses were blocked by tetrodotoxin pretreatment. 5 These data suggest that the SO receives inputs from duodenal mechano and/or stretch receptors resulting in excitatory or inhibitory responses, with the excitatory response dominating. These findings support the role for the SO-duodenal reflex in preventing duodenobiliary/pancreatic reflux during periods of elevated duodenal activity.