The growth of the first hair coat in male mice was studied during administration of epidermal growth factor (EGF). Injections of 1 or 4 μg EGF/g body weight for 14 consecutive days from birth resulted in the development of curved overhairs (monotrichs), caused a retardation in rate of growth in length of hair and a reduction in hair diameter and length of follicle bulb. Growth rate partially recovered after cessation of EGF treatment. However, some of the effects produced by injections of EGF during the formation of the first coat were detected in the second and third generations of hair. Since EGF also retarded rate of body growth, we compared the effect of EGF on hair growth with that of restricting food intake in neonatal mice during the development of the first coat. Hair growth was slowed in underfed animals but the effects were less marked than those found in EGF-treated mice of similar body weights.