We have investigated aberrant methylation of CpG nucleotides (CpG sites) and gene expression of c-myc during hepatocarcinogenesis induced by a choline-deficient, L-amino acid-defined (CDAA) diet in rats. Male Fischer 344 rats, 6 weeks old, were continuously given a CDAA diet for 50 and 75 weeks and then killed. Macroscopically detectable nodules, which were histologically confirmed to be hyperplastic nodules (HNs) or well-differentiated hepatocellular carcinomas (HCCs), were dissected free from the surrounding tissue. Normal control liver was obtained from 6-week-old rats. Methylation of CpG sites of the c-myc gene was investigated in bisulfite-treated DNA isolated from normal liver, HNs and HCCs. All 33 cytosines in the 5'-upstream region of the c-myc gene were fully methylated in control liver and the 4 HNs. In contrast, these cytosines were completely unmethylated in 5 HCCs. Examination of the c-myc expression by reverse transcription-polymerase chain reaction (RT-PCR) analysis also showed a marked increase as compared to the low levels in normal livers and HNs. These results suggest that hypomethylation of the c-myc gene might play a critical role in malignant transformation from HN to HCC during CDAA diet-induced hepatocarcinogenesis in rats.