Multiple occurrence or recurrence after transurethral resection is an important characteristic of superficial bladder tumors. To study bladder carcinogenesis, we focused on detection of telomerase activation, which was investigated in several human cancers, including bladder tumors. We experimentally examined the telomerase activity during bladder carcinogenesis, especially in precancerous lesions, induced by N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN) in rats. Male Wistar rats were given 0.05% BBN in water from the age of 8 weeks to 24 weeks. Subgroups were euthanized at 4, 8, 10, 12, 18, and 24 weeks after BBN administration. Using the stretch PCR method, telomerase activity was semiquantified in exfoliated bladder epithelial cells. In addition, telomere length in each subgroup was measured by southern hybridization for the terminal restriction fragment using a (TTAGGG)(4) probe. Statistical analyses were performed using analysis of variance and Fisher's PLSD test. Epithelial cells of normal bladder in the control groups and those of diffuse hyperplasia, which was a reversible change at 4 weeks, expressed no telomerase activity. In contrast, telomerase activity significantly increased in the stage after nodular hyperplasia, an irreversible change at 8 weeks, then elevated with carcinogenesis. However, telomere length was still preserved by the 12th week, and was shortened at 18 and 24 weeks. These results suggest that telomerase activation is probably induced independent of telomere shortening during bladder carcinogenesis in the rat, and might be a biological tumor marker of irreversible preneoplastic lesions, which evolve into bladder tumors in the rat.