AbstractThe structure of the epidermis and hair‐follicle bulbs and the proliferative activities of their component cells were studied in the midside skin of male mice treated with epidermal growth factor (EGF) or saline during formation of the first coat (days 0–21). The epidermis was thickest at birth, but in control animals became progressively thinner as cell size and the number of layers of granular and cornified cells were reduced. EGF treatment from birth resulted in a thickening of the epidermis at 5 days of age, and at 5 and 8 days the histological appearance was strikingly similar to that on the day of birth. At 12 and 21 days the structure of the epidermis of EGF‐treated mice more closely resembled that of contemporaneous controls. The mitotic and labeling indices of the basal cells of control epidermis declined throughout the sampling period from peak values on the day of birth. By contrast, these indices were maintained at birth levels in EGF‐treated mice for 8 days before declining to approximate those of controls at 12 and 21 days.Hair growth rate was inhibited and hair diameter reduced in EGF‐treated mice (Moore et al., 1981a). These observations were reflected in changes in the follicle bulb. Both the growth of the bulb and the increase in numbers of bulb cells observed during the early part of the anagen phase were inhibited by EGF. However, neither the size of the dermal papilla nor the numbers of papilla cells were significantly altered in treated animals. The mitotic index of the bulb cell population declined during the sampling period in both experimental and control groups. However, the labeling index of bulb cells of EGF‐treated mice was significantly increased over contemporaneous control values on days 8 and 12. Rather than stimulating epidermal growth during the early postnatal period, these observations indicate that EGF delays the normal process of skin development by maintaining the proliferative and differentiation processes active in the cell populations at the time of birth. As a consequence of this, follicle development and hair growth ar