Precancerous lesions and carcinomas were produced in rat tongues by oral administration of 0.001% 4-nitroquinoline 1-oxide (4NQO) in drinking water, and the biological characteristics and tumor kinetics were studied. Bromodeoxyuridine labeling index (BrdU L.I.) was determined immunohistochemically, and the nuclear DNA content was measured by DNA cytofluorometry. The BrdU L.I. was lowest in normal epithelium (3.49 +/- 0.49%), higher in papillomas (8.24 +/- 1.76%) and highest in squamous cell carcinomas (13.36 +/- 4.40%). Normal lingual epithelial cells, papillomas and 18 of the 25 squamous cell carcinomas showed a diploid pattern, while 7 squamous cell carcinomas showed a diploid plus tetraploid pattern. The correlation between polyploidization and tumor progression was significant, but the correlation between BrdU L.I. and tumor progression was not significant. The results suggest that while polyploidization of tumors increases with tumor progression the growth rate of tumors does not necessarily do so.