An experimental model of anomalous arrangement of the pancreaticobiliary ducts (APBD) was produced by pancreaticocholecystostomy performed in 29 mongrel dogs. Our purpose was to study carcinogenesis of the extrahepatic biliary tract by DNA ploidy analysis with cytofluorometry. The amylase level in the bile was elevated in all 25 dogs tested except the controls. The phospholipase A2 level in the bile was elevated in all selected dogs except the controls. Common bile duct (CBD) dilatation was found in 23/29 (79%) of the dogs, and biliary stones were found in 3/29 (10%) dogs. Inflammatory changes were observed microscopically in all specimens except those from the controls. Intramural glandular structures were found in 17/25 (68%) of gallbladder (GB) specimens and 10/25 (40%) of CBD specimens; goblet cells were found in 7/25 (28%) of GB specimens and 2/25 (8%) of CBD specimens. In the controls neither glandular structures nor goblet cells were observed except for two GB specimens showing mild cholecystitis. Cytofluorometry showed 21% GB and 7% CBD diploidy, 69% GB and 65% CBD low ploidy, 10% GB and 28% CBD high ploidy patterns of histograms. These results show that, APBD may be central to high risk condition or play a key role to develop atypical biliary tract epithelium and DNA ploidy abnormality with or without biliary duct dilatation.