In order to elucidate the role of point mutation of the K-ras gene in the tumorigenetic process of lung tumors, an experimental model of lung lesions in mice induced by the administration of urethane was used. A total of 135 B6C3F1 male mice, 6 weeks old, were given urethane in the drinking water at 0, 6, 60, 600 or 1200 ppm, and were then killed after varying periods of time. The lung lesions were histologically characterized as hyperplasia, adenoma and adenocarcinoma. Point mutations in codons 12 and 61 of the K-ras gene were detected by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) and confirmed by using dideoxy sequencing analysis. K-ras gene mutation was identified in 9 (23.7%) of 38 lesions classified as hyperplasia, 31 (46.3%) of 67 adenomas, and 3 (50%) of 6 adenocarcinomas. The most frequent mutation was an AT-to-TA transversion at the second base of codon 61 and this pattern accounted for 65% of the three mutant forms observed. These results suggest that the point mutation of K-ras gene is involved in all stages of mouse lung tumorigenesis, i.e., activation of this gene can also influence the later stages of lung lesions.