Ki‐ras and p53 genes are involved in human lung carcinogenesis; however, the role of these genes in experimental lung tumors is not well known. In our study, the CBA/J mouse strain was used to investigate the presence of Ki‐ras and p53 alterations in lung carcinogenesis of spontaneous tumors and tumors induced with high and low doses of urethane (ethyl carbamate). To study the presence of these alterations in the early stages of lung carcinogenesis and in very small lung tumors, restriction fragment length polymorphism and single‐strand conformation polymorphism analyses were performed on polymerase chain reaction–amplified DNA from microdissected tumoral and normal lung samples. Ki‐ras gene mutations in codons 12 and 61 were detected in all types of lung lesions, even in small and preneoplastic lesions, and their incidence increased with progression from lung hyperplasias (18%) to adenomas (75%) and to carcinomas (80%). Urethane exposure, in both high and low doses, increased the incidence of Ki‐ras mutations in lung tumors, especially in adenomas. The presence of Ki‐ras gene mutations in very small urethane‐induced lung tumors and the absence of hyperplasias among the treated‐group lesions may indicate that urethane accelerates tumoral progression. No p53 mutations were detected in exons 5–8 in any of the epithelium‐derived lung tumors. Only one p53 mutation in exon 5 was found in a spontaneous lymphoma. Therefore, p53 mutations do not seem to cooperate with Ki‐ras gene mutations or represent an alternative molecular pathway in murine carcinogenesis. Mol. Carcinog. 21:251–260, 1998. © 1998 Wiley‐Liss, Inc.