Cohorts of 4- to 5-wk-old female Fischer 344 rats received four biweekly 1.5-mg doses of N-methyl-N-nitrosourea (MNU) intravesically and were sacrificed at various intervals. By 13 wk after initiation of the carcinogen, all animals have flat epithelial atypia and/or papillary transitional cell bladder carcinomas, and 67% of the lesions are histological Grade II or III. By 20 wk, 83% have gross bladder wall muscle-invasive tumors that eventually kill the host. There was no gross evidence of visceral metastases in any animal. This rat model of transitional cell carcinoma of the bladder is useful because: (a) all animals develop progressive neoplastic changes in situ within 4 mo after initiation of MNU treatment; (b) these lesions progress to grossly detectable bladder tumors which invade the bladder wall and kill the host; (c) this full progression of bladder epithelial cells from atypical hyperplasia through flat carcinoma in situ to transitional cell carcinoma occurs at discrete time points; (d) the histology of the grossly detectable tumors is that of invasive transitional cell carcinomas; and (e) no leukemias, breast cancers, lymphomas, or other non-bladder tumors are induced. Six MNU-induced bladder wall-invasive tumors were karyotyped, and all tumors were diploid with 42 chromosomes. Three of the tumors had apparently normal karyotypes, while three tumors had karyotypes containing one or more cytogenetic structural markers. One of these markers (i.e., 8p+) was observed in two of the three tumors. The level of expression of total ras p21 (N-, Ki-, and Ha-ras p21) and codon 12-mutated c-Ha-ras p21 (i.e., glycine to glutamic acid mutation in codon 12) in a series of these MNU-induced bladder tumors was determined by Western blot analysis. No increase in the total ras p21 nor any expression of codon 12-mutated c-Ha-ras p21 was detected in any of these tumors.