c-myc expression, amplification, and rearrangement were studied in neoplastic hepatic nodules of rats after diethylnitrosamine treatment, in hepatocellular carcinomas induced by N-methyl-N-nitrosourea, and in tumorigenic liver cell lines derived from rat hepatocytes transformed by diethylnitrosamine in vivo. Steady-state levels of c-myc transcripts, as measured by Northern blot hybridization, were slightly increased in neoplastic hepatic nodules and showed high levels in some hepatocellular carcinomas and some tumorigenic liver cell lines. DNA of the samples was analyzed after restriction nuclease treatment by Southern blot hybridization, using different probes specific for the three exons of the c-myc gene. The results suggest rearrangements of the complete gene and/or amplification in some cases. Rearrangements include the 5'-part of the first exon and a stretch upstream c-myc supposed to contain control elements of c-myc expression. Aberrations of this kind are most pronounced in advanced metastasizing hepatocellular carcinomas and highly tumorigenic, metastasizing liver cell lines. Heterogenicity of genomic alterations and c-myc transcript levels were found among different samples of the same hepatocellular carcinoma indicating subclonal diversification.