We have previously shown that rats fed raw soya flour (RSF) for more than four months develop hyperplastic foci of pancreatic acinar cells, which undergo malignant change if feeding RSF is continued throughout the life of the animals. The tendency to undergo malignant change is augmented by the additional use of a genotoxic carcinogen such as azaserine. The present study has sought to examine the reversibility of the focal neoplastic change in the pancreas. Rats fed RSF for 24 weeks and then given a diet not containing soya flour (NSC) had a normal pancreas when killed after 60 weeks of study. When RSF was fed for only 36 weeks, however, some of the rats developed pancreatic cancer even though the diet had been switched to NSC. Similarly, while azaserine in the dose used in the present study does not produce pancreatic cancer in our strain of Wistar rats, coincident administration of RSF for 12 weeks (but not for six weeks) resulted in progression to pancreatic adenoma. Although change from RSF to NSC after 30 weeks resulted in rapid reduction in pancreatic weight and content of RNA, neoplastic foci persisted and became frankly malignant. We conclude that phenotypic reversion to normal of the RSF diet- and azaserine-treated rat pancreas is only possible if RSF alone is fed continuously for not more than about 24 weeks or six weeks if the rats have been exposed to a pancreatic initiating carcinogen.