We have established a transgenic rat line carrying three copies of the human c-Ha-ras proto-oncogene with its own original promoter region, Jcl/SD-TgN(HrasGen)128Ncc (Hras128) rat. c-Ha-ras protein from expression of transduced and endogenous c-Ha-ras genes could be detected in the bladder epithelium of untreated transgenic rats. To examine their susceptibility to N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN)-induced urinary bladder carcinogenesis, male transgenic and wild-type littermates were treated with 0.05% BBN in their drinking water for 10 weeks and then killed at week 20. The numbers and volumes of total macroscopic bladder tumors including both transitional cell papillomas and carcinomas (TCC) per rat were much greater in Hras128 rats than in their wild-type counterparts. The numbers of carcinomas per rat were also significantly greater in Hras128 rats. Two cases of TCC exhibiting invasion of the bladder muscle layer, which is extremely rare in the wild-type animals under the experimental conditions used, were also observed in Hras128 rats. The GGC-->GAC mutations at codon 12 of the transgene were observed in only two TCC out of 21 bladder tumors (9.5%), assessed by RFLP analysis and direct sequencing. SSCP analysis did not show any endogenous c-Ha-ras gene mutations. One of 25 tumors (4.0%) in wild-type rats had an endogenous c-Ha-ras gene mutation at codon 12 that was detected (GGA-->GAA) by single-strand conformation polymorphism and direct sequencing. These results indicate that the Hras128 rat is highly susceptible to BBN carcinogenesis and may be utilized as a rat model for analysis of bladder tumor development. The mutation findings indicate that the enhanced tumor development is not primarily due to mutations occurring in the transgene.