The function of the 5'-flanking region of a rat glutathione S-transferase Ya subunit structural gene has been examined in homologous and heterologous cells. By using the 5'-flanking region of the Ya subunit gene fused to the structural gene encoding chloramphenicol acetyltransferase, we have identified two cis-acting regulatory elements in the upstream region of the Ya gene. One element is required for maximum basal level expression in homologous cells, whereas maximum basal level expression in homologous cells, whereas the second element is required for inducible expression of the Ya gene by planar aromatic compounds such as beta-naphthoflavone. The cis-acting element required for inducible expression of the Ya gene by beta-naphthoflavone is functional only in cells with normal dioxin receptors.