Exposure to X radiation is associated with a decline in the proliferative activity of the liver, but the molecular mechanism(s) is not well understood. We investigated whether exposure to X radiation is involved in functional changes in the epidermal growth factor (EGF) receptor (EGFR), thereby causing a reduction of EGF-induced DNA synthesis using periportal hepatocytes (PPH) and perivenous hepatocytes (PVH), which differ in their proliferative activity. X radiation dose-dependently decreased DNA synthesis in both subpopulations. The rate of decline in the DNA synthesis was greater in PPH than in PVH, but the zonal difference disappeared after exposure to 10 Gy X radiation. [¹²⁵I]EGF binding studies indicated that high-affinity EGFRs in both subpopulations were down-regulated after X irradiation. Furthermore, EGF-induced EGFR dimerization and phosphorylation at Y1173 in both subpopulations were down-regulated after X irradiation, and the rate of decline was greater in PPH than in PVH. In contrast, phosphorylation at Y845 after EGF treatment was dose-dependently up-regulated after X irradiation in both subpopulations. These results suggest that the X-radiation-related decline in EGF-induced DNA synthesis is caused at least partly by the modification of EGFR function.