In this study, the effects of dietary fat and selenium on the development of putative preneoplastic foci induced by aflatoxin B, were examined. Rat hepatic y‐glutamyl transpeptidase (GGT) activity, selenium‐dependent glutathione peroxidase activity, glutathione, and total lipid were analyzed 3 and 13 weeks after the start of dietary and carcinogen treatment. Results indicate that high dietary fat has an enhancing effect on the development of preneoplastic foci. A high‐fat, selenium‐deficient diet resulted in an increase in foci development, but the increase was not significant. An interaction between aflatoxin and the two nutrients was also observed in liver glutathione content and GGT activity at Week 3.