In this study the relationship between the gallbladder pressure (GBP) and sphincter of Oddi (SO) activity was studied in dogs. The GBP was modulated by varying the volume content in a reservoir placed within the gallbladder at ranges corresponding to physiological GBP ranges. For SO manometry a low compliance high fidelity system was used. Firstly the influence of alternating gallbladder emptying and filling on SO activity was studied. A clear relationship was seen in all dogs so that an increase in GBP inhibited SO activity. Subsequently, stepwise increases in GBP from 0 to 38 mm Hg in consecutive 10-min measuring periods showed a definite inverse relationship betweeen GBP and SO frequency, baseline pressures and wave amplitude. It is concluded that a reflex relationship between the gallbladder and the SO exists, operating within physiological ranges of GBP so that increased GBP inhibits SO activity. The importance of this relationship in bile flow dynamics and the possible consequence in the postcholecystectomy state is mentioned.