The effect of partial pancreatectomy (PP) on the pancreatic carcinogenicity of N-nitrosobis (2-oxopropyl)amine (BOP) was investigated in Syrian golden hamsters by subcutaneous injection of a single dose of BOP (20 mg/kg, body weight) given 30 minutes after (Group 1), 1 week after (Group 2), or 1 week before 70% PP (Group 3). Additional groups consisted of animals with PP alone (Group 4), sham operation (laparotomy) followed 30 minutes later by BOP treatment (Group 5), and BOP treatment only (Group 6). The experiment was terminated 46 weeks after BOP administration in each group. The pancreas and extrahepatic bile ducts, including the common duct and gallbladder, were examined histologically. Tumor patterns were compared in hamsters with PP and in the corresponding segments of the pancreas in BOP-treated control groups. The pancreatic cancer incidence was highest (31%) in Group 2 and lowest in Group 1 (3%), a difference that was statistically significant (P less than 0.01). Also, a statistically highly significant larger number of tumors occurred in Group 2, compared with group 1, 3, or 5 (P less than 0.0005). In a comparison of the number of carcinomas per tumor-bearing hamster, there were greater numbers of carcinomas in Group 2 (2.6 carcinomas) than in Groups 1, 3, 5, and 6 (1.0, 1.0, 1.3, and 2.6 tumors, respectively). Moreover, pancreatic tumors in Group 2 hamsters were larger (average diameter, 10 mm) than in Group 1 (4 mm), Group 3 (3.5 mm), Group 5 (4 mm), and Group 6 (average, 9mm). The incidence of extrapancreatic tumors did not vary among the PP groups but was equally lower than those in BOP-treated control groups. The data indicated BOP carcinogenesis was inhibited by surgery (regardless of whether PP was per formed) when the carcinogen was given 30 minutes after the surgery but was significantly enhanced when BOP was administered 1 week after PP. The possible reasons for these conflicting results are discussed. Morphologically all tumors were of ductular, ductal, and mixed ductular-insular patterns and most developed at the resected margins, where proliferation of islets, ducts, and ductules, but not of acinar cells, occurred. The results confirm our view that the ductal and ductular cells are the progenitor cells for BOP-induced pancreatic tumors in hamsters.