Well-differentiated mammary carcinomas carrying mutated Ha-ras-1 oncogenes arise frequently in pubescent rats exposed to the direct-acting methylating agent N-methyl-N-nitrosourea (MNU). When these tumors are serially transplanted, they acquire more aggressive phenotypes. To determine the genetic alterations underlying local invasion, hormone independence, and metastasis, we studied alterations in the Ha-ras-1, p53, and mdm2 genes in successive generations of tumors passaged in intact or ovariectomized rats. Although previous studies have shown that selective amplification of the mutant Ha-ras-1 allele correlates strongly with the acquisition of hormone independence, we found that the acquisition of an invasive phenotype did not depend on mutational activation or amplification of Ha-ras-1. Mutations in the p53 gene were rare. Of a total of 120 primary, locally invasive, hormone-independent, and metastatic tumors tested for mutations in exons 4-9 of the p53 gene, only one mutation was detected in the later passages of an invasive tumor line. No gross gene alteration or amplification was seen in mdm2, a negative regulator of p53 transcription. Thus, the p53 gene is an infrequent mutational target, and amplification of the mdm2 gene does not appear to play a role in initiation or progression of rat mammary tumorigenesis.