Epidermal growth factor (EGF) was present at a mean concentration of 266 +/- 18 (S.E.M.) pmol/mg wet tissue in the submandibular gland of 3-month-old male mice; it was also present in plasma at a concentration of 364 +/- 149 pmol/l. Sialoadenectomy (removal of the submandibular glands) decreased the plasma EGF content to undetectable levels (less than 16.5 pmol/l), lowered the concentration of EGF in the skin from 1.22 +/- 0.11 to 0.47 +/- 0.08 fmol/mg wet tissue and reduced the thickness of the epidermis from 28.9 +/- 2.7 to 11.0 +/- 0.8 micron in 3 weeks (P less than 0.001). Epidermal growth factor antiserum given to sialoadenectomized mice further decreased the thickness of the epidermis to 8.3 +/- 0.6 micron. No appreciable change was observed in the dermis and subcutaneous tissue. In sialoadenectomized mice, replacement of EGF prevented the decrease in thickness of the epidermis in a dose-dependent manner when started immediately after the operation. Treatment with EGF also effectively restored the normal morphology of the epidermis when its thickness had declined to its lowest level. These results suggest that EGF plays a physiological role in the maintenance of the epidermis.