The effects of intervention by diets with high or low levels of dietary fat on the development of preneoplastic pancreatic lesions were examined. Wistar rats were treated ip at 14 days of age with a 30-mg/kg dose of L-azaserine [CAS: 115-02-6; diazoacetate serine (ester)] and weaned onto the test diets. Animals fed 5% corn oil had fewer preneoplastic lesions compared to animals fed 20% corn oil throughout the 4-month posttreatment period. The strong response observed in rats fed 20% corn oil could be markedly reduced by intervention with a 5% corn oil diet halfway through the posttreatment period. Similarly, the low response in animals fed 5% corn oil could be markedly elevated by intervention with a high-fat diet. These results provide evidence for the hypothesis that tumor development may be modified by dietary means.