Di(2-ethylhexyl) phthalate (DEHP) and di-n-octyl phthalate (DOP) are nongenotoxic carcinogens found in drinking water, which induce mainly liver tumors. The mechanism of tumorigenesis by these chemicals remains to be elucidated. The present study was undertaken to test whether any known oncogenes are activated in DEHP- and DOP-induced tumors. The authors examined tumor and chemically treated rat liver RNAs for elevated expression of 10 different oncogenes: c-myc, c-fos, c-Ha-ras, c-Ki-ras, c-raf-l, c-jun, erbA, v-bas, v-sis and v-src. As controls, they used tissue-specific genes such as albumin, cytochrome P450, aldehyde dehydrogenase, and ornithine decarboxylase. Their results indicated that the expression pattern of oncogenes and other genes in chemically treated livers or tumors was not greatly different from that of controls, although in individual livers, multiple changes in transcription were detected. Recently, the authors isolated two genes from a hepatoma cell line by subtraction hybridization.