It is common knowledge among surgeons that calculi in the common bile duct are frequently overlooked. Consequently, the common duct is explored at the time of cholecystectomy in an increasing percentage of cases. The exploration is not considered complete unless the patency of the papilla of Vater is proved, and this step is often combined with systematic instrumental dilation of the papilla. The methods used for dilation differ somewhat in the type of instrument and in the amount of distention recommended. The dilators employed have been in most instances either soft woven catheters1 or metal probes with olive tips (Bâkes).2 It is the purpose of this paper to describe a series of experiments planned to reproduce as nearly as possible in the experimental animal the conditions of the operation. The immediate and late effects of dilation with both types of instruments have been studied, and the influence of