In an earlier experiment we found that 1-nitropyrene treatment causes an increase in the expression of certain onco/suppressor genes in different organs of CBA/Ca mice. In order to further study the kinetics and significance of these gene expression changes, we determined the effect of 1-nitropyrene treatment on the expression of c-myc, p53, Ha-ras, N-ras, and Ki-ras genes, 24 hours after treatment. Expression of the ras family did not change during the studied interval, while elevated expression of c-myc and p53 genes was observed in the spleen, bone marrow and lymph nodes (only c-myc in the latter). The results suggest a different pattern for the involvement of the ras genes in 1-nitropyrene-caused carcinogenesis, and also underlines the differences in the organ specificity of chemical carcinogens in humans and in experimental animals. In the present study, we also confirmed the in vivo applicability of early gene expression changes as biomarkers of carcinogenic exposure.