We induced pancreatic adenocarcinomas in Long-Evans rats by placing crystals, 2-3 mg, of 7,12-dimethylbenz[a]anthracene (DMBA) in a 2- to 3-mm incision in the "head" of the pancreas approximately 1 cm from the duodenum. The incisions were closed with one or two silk sutures. The animals were killed 4-10 months after DMBA implantation, and nodules were removed and routinely prepared for light and/or electron microscopic study. Histologic organization varied from normal, through areas of tubule-like structures, to sheets of pleomorphic tumor cells. Electron microscopic study of tumor cells revealed large electron-lucent nuclei that frequently had irregular outlines and prominent nucleoli. The predominant feature of the cytoplasm was abundant rough endoplasmic reticulum. Zymogen granules were rare. Adjacent cells sometimes were jointed by an apical junctional complex to form a lumen into which projected irregular microvilli. A basal lamina sometimes occurred at the bases of the tumor cells. The fine structural similarity of these tumor cells to acinar cells was noted.