Fragments of normal rat liver and nodular hepatocellular lesions produced by feeding rats 0.02% N-2-fluorenylacetamide for 20 weeks were transplanted into the inguinal mammary fat pad of syngeneic male rats. Five of 29 fragments of normal liver survived in the transplant site for up to 32 weeks after transplantation. Six of 20 nodules classified as neoplastic nodules and 2 of 5 with atypicality persisted and retained their morphologic abnormalities. Persisting fragments of normal liver accumulated iron when the recipient rats were iron-loaded by the feeding of 8-hydroxyquinoline and ferrous gluconate, but the nodule cells remained resistant to iron accumulation. Mitotic figures were present in nodule cells, but progressive growth did not occur. Four carcinoma transplants readily gave rise to tumors. Three of the tumors that developed in the transplant site from carcinomas were well-differentiated adenocarcinomas with mucin production. It was concluded that rat liver neoplastic nodules are phenotypically stable but differ from liver carcinomas in their biologic behavior in this system.