A canine model of acute cholecystitis was used to determine the mechanism of opacification of the gallbladder wall by infusion tomography. The experimental data indicate that opacification of the thickened wall of the gallbladder in acute cholecystitis is a function of hypervascularity and transport of the radiopaque material across the capillary wall by polymorphonuclear cells. Tissue-fluid equilibration of the contrast agent may also play a role.