Epidermal growth factor (EGF) and EGF-related growth factors are present in the urine, and EGF has been identified as a urinary component that enhances urinary bladder tumor formation in rats. Neu oncogene encodes a cell surface receptor similar to the EGF receptor and is known to be activated by a point mutation of DNA that encodes the transmembrane domain of the neu protein (p185). In this study, we examined the possible mutational activation of neu oncogene in 50 urinary bladder transitional cell carcinomas (TCC) induced in F344 rats by the following carcinogenesis models: (i) 0.05% N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN) (4 weeks)----3% uracil (20 weeks); (ii) 0.2% N-[4-(5-nitro-2-furyl)-2-thiazolyl]formamide (FANFT) (6 weeks)----5% sodium saccharin (72 weeks); and (iii) N-methyl-N-nitrosourea (MNU) 20 mg/kg body wt, i.p. twice per week for 4 weeks----3% uracil (20 weeks). The DNA sequence around the transmembrane domain of neu gene was amplified by PCR and sequenced. The results showed no mutation within the examined DNA sequences, indicating that neu oncogene is not activated by a point mutation in the transmembrane domain in urinary bladder carcinomas induced by BBN, FANFT or MNU.