The influence of dietary manipulations on fatal neoplasms in male Fischer 344 rats was assessed. Particular attention was paid to leukemia and pituitary adenoma because they are the most common potentially fatal neoplasms that occur in this rat strain. The only dietary manipulation which depressed the mortality rates due to neoplastic disease was that involving a reduction in energy intake. Reduction of mineral or protein or fat intake without a reduction of energy had, at most, marginal effects on fatal neoplastic disease.