The effect of daily topical minoxidil treatment on hair growth was investigated in eight neonatal hairless pups. After 21 days of minoxidil treatment, hair growth occurred in the minoxidil-treated areas of each neonatal hairless pup. The numbers of hairs in the treated areas increased as compared with that in the areas treated with vehicle only. Histological findings revealed that the skin treated with minoxidil contained many hair follicles derived from epidermal ingrowths (hair germs) projecting into the dermis. These hair follicles showed various stages of neofolliculogenesis. After 31 days of treatment, terminal hair growth was observed. In the minoxidil-untreated areas, epidermal ingrowths remained undifferentiated. These results revealed that hairless dogs developed hair growth reactions following minoxidil-treatment when such treatment is applied in the early neonatal period. Neonatal hairless pups are a useful model for evaluating the effectiveness of hair growth stimulators.