Raw soya flour (RSF) feeding to rats produces pancreatic hypertrophy and hyperplasia, and, if sufficiently prolonged, the spontaneous development of pancreatic neoplasms and the potentiation of pancreatic carcinogens. With continuous exposure to RSF the threshold dose for pancreatic growth is approximately 20% RSF, but the threshold for the other effects has not been defined. If 100% RSF is fed for less than 24 weeks continuously the effects on the pancreas are completely reversible, but feeding for longer than this leads to irreversible progression to pancreatic adenoma and carcinoma over the next 50-70 weeks. Repeated alternation of the diet between 100% RSF and rat chow for prolonged periods leads to effects on the pancreas at least as marked as those seen with continuous RSF feeding. This occurs with RSF feeding for periods as short as 2 days out of 7. The effects seen in the rat must be applied to human nutrition with caution, but it is suggested that infants fed soya based milk substitutes and fad dieters who frequently radically alter dietary composition may be more susceptible to spontaneous or carcinogen-induced pancreatic neoplasms.