Previous studies have shown that epidermal growth factor (EGF), a peptide of M.W. 6045, can specifically inhibit in organ culture the cessation of DNA synthesis and programmed cell death that normally occur in the presumptive fusion zone (PFZ) of the secondary palatal epithelium. The aim of this study was to determine if EGF acts directly on the epithelium to exert its effect and if there is a requirement for the underlying mesenchyme. Palatal processes from 13- and 14-day Swiss Webster embryonic mice were enzymatically separated into epithelium and mesenchyme which were then cultured alone or in transfilter recombination for up to 72 h. Tissues were examined by transmission- and scanning-electron microscopy and DNA synthesis was monitored autoradiographically using [³H]thymidine incorporation. In isolated epithelium cultured in control medium, cell death occurred in the PFZ and DNA synthesis did not occur in the oral and nasal epithelial regions. EGF (20– 50 ng/ml) did not prevent cell death in the PFZ and failed to stimulate DNA synthesis in the isolated epithelium; EGF, however, did have an effect on epithelial cell morphology. In the presence of mesenchyme and EGF, there was extensive proliferation in the entire epithelium and cell death within the PFZ was not evident. The results indicate that the stimulation of DNA synthesis in the palatal epithelium by EGF requires the presence of the underlying mesenchyme and that EFG alone is not sufficient to inhibit programmed cell death within the PFZ of the isolated palatal epithelium.