Groups of male Fischer 344 rats were chronically fed semipurified choline-devoid or choline-supplemented diets, high in fat (15%), and containing or not containing 0.06% phenobarbital. Atypical acinar cell nodules were observed in the pancreas of the rats, irrespective of the diet fed, with incidences varying from 38% to 100% in the various groups. No consistent differential effects of the dietary treatments on the incidence and growth of the nodules were evident, even though the diameter of the nodules tended to be greater in some of the groups fed the basal choline-devoid diet. The vast majority of the nodules were of the acidophilic type. More advanced pancreatic acinar cell lesions were observed in a few of the rats. Since the rats were not exposed to a chemical carcinogen(s), development of the nodules and of the more advanced lesions, even in rats fed the control diets, was most likely due to evolution of endogenous (spontaneous) initiated pancreatic cells, promoted primarily by the feeding of semipurified diets with a high fat content.