Modulation of DNA synthesis by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was studied in primary cultures of hepatocytes and in rat liver epithelial cells (WB-F344) to develop models for studies on the interactions between the activated Ah receptor and cellular growth control. In hepatocytes TCDD either positively or negatively modulated EGF-stimulated DNA synthesis. In the presence of ethinylestradiol 10(-12) M TCDD moderately increased EGF-stimulated DNA synthesis (approximately 30%). In contrast, 10(-9) M TCDD in the absence of ethinylestradiol decreased DNA synthesis (approximately 30%). Analysis of variance revealed that the TCDD effects were highly significant. The response of 'early genes' of the jun/fos family and the corresponding proteins was also studied under these two conditions. In agreement with the DNA synthesis data, the level of c-Jun was increased or decreased in nuclear extracts. Furthermore, DNA binding of Jun/Fos proteins, including c-Jun and Fra-1, was decreased under conditions of mitoinhibition, while the level of Fra-1 in nuclear extracts was increased. In WB-F344 cells TCDD treatment for 44 h increased DNA synthesis 2- to 3-fold in comparison with controls, based on measuring [3H]thymidine incorporation into DNA or on determining the nuclear labeling index with bromodeoxyuridine. This effect is probably due to inhibition of high density growth arrest by TCDD. The proposed cellular models may be useful to elucidate the interactions between the activated Ah receptor and signaling pathways of growth homeostasis.