The nuclear pore density and area were measured on freeze-fractured nuclei of ACI/N rat liver altered foci, adenomas and carcinomas induced by 2-acetylaminofluorene, and compared with those of normal hepatocytes. The pore density of nuclei from these preneoplastic and neoplastic lesions was significantly higher than that of hepatocytes, but there was no difference between lesions. The area of nuclear pores of the focus cells did not differ from normal hepatocytes, whereas the areas of pores of adenoma and carcinoma cells were increased. Moreover, the nuclear pore area of carcinomas was significantly greater than that of adenomas. These results suggest that some changes may occur in nuclear pores in the progress of tumorigenesis.