The effects of epidermal growth factor (EGF) on the production and secretion of hCG and human placental lactogen (hPL) by cultured placental tissues were investigated in relation to immunohistochemical measurements of cellular EGF receptor levels in the placenta. Explants of trophoblastic tissues obtained from normal early and term placentas were cultured in the presence or absence of EGF (100 ng/mL) with or without processing inhibitors (bacitracin, 1 mg/mL; colchicine, 100 microM; chloroquine, 100 microM) for 5 days, with EGF present for the first 2 days. Addition of EGF to the medium increased the release of hCG, hCG alpha, and hPL by the cultured early placental tissues. This EGF-stimulated hCG, hCG alpha, and hPL release was markedly inhibited by concomitant treatment with processing inhibitors. The time course of EGF effects indicated that the EGF-stimulated increase in hCG alpha secretion required a lag period of approximately 1 day, whereas significant increases in hCG and hPL secretion became apparent only after 3 days of EGF treatment. By contrast, in term placental tissues EGF stimulated only hCG alpha and hPL release, with a lag period of approximately 3 days. A possible direct action of EGF on the cultured placental tissues was reinforced by the immunohistochemical demonstration of EGF receptors in the placenta. When determined using the avidin/biotin immunoperoxidase method with monoclonal antibody to the mouse EGF receptor, EGF receptors were found predominantly on the syncytiotrophoblasts. Immunohistochemical measurements of cellular EGF receptor levels in the syncytiotrophoblasts revealed remarkably higher levels in early placenta compared to those in midterm and term placentas. Since EGF is likely to interact with its receptor, the lesser biological effects of EGF in cultures of term placental tissues may be due to the lower cellular EGF receptor levels in term placenta. These results demonstrate that EGF, via its receptors on the syncytiotrophoblasts, stimulates the release of both hCG and hPL in normal early placenta. They also suggest that EGF may play a significant role in the induction and regulation of the differentiated function of trophoblasts.