The changes of the biliary tree following distal bile duct obstruction and its release were confirmed by biliary scintigraphy and monitored by serial ultrasonography, computed tomography, and values of serum bilirubin and alkaline phosphatase in 14 mongrel dogs. The degree and rate of biliary dilatation were independent of cholecystectomy. The most rapid rate of extrahepatic dilatation occurred within the first 48 hours, while dilated intrahepatic ducts were first recognized three to six days after obstruction. Following release of the obstruction, the degree and rate of resolution of the biliary dilatation were independent of the duration of ligation (one vs. two weeks) and cholecystectomy. The dilatation resolved slowly. Dilated intrahepatic ducts were recognized for as long as eight to 13 days, while extrahepatic biliary dilatation was still present for 13 weeks, at which time the experiment was terminated. It is postulated that the extrahepatic biliary dilatation will approach a plateau approximately one month after total biliary obstruction. It appears that if the obstruction lasts more than one week, it results in irreversible damage to the elasticity of the extrahepatic ducts. Thus, after release of the obstruction, serial biliary imaging is indicated until a new baseline of the biliary tree diameter has been established.