By means of skin organ culture, some biological characteristics of hair follicle elongation were examined. When skin sections from 6-day-old C3H mice were cultured, spontaneous elongation of hair follicles was maintained. Without insulin, hair follicle elongation was poorly maintained irrespective of the presence of serum at 20%. Insulin could be replaced by IGF-I at 100 ng/ml. During in vitro elongation of hair follicles, bromodeoxyuridine was incorporated into germinal epithelial cells around dermal papillae. Skin sections from 4-week-old C3H mice did not show hair follicle elongation in complete medium. However, when 0.5 mM minoxidil was added to the medium, concentration dependent thickening and elongation of hair follicles was observed. In contrast, in vitro elongation of newborn pelage hair follicles was not enhanced by minoxidil. These results suggest that this system with the use of skin sections from 4-week-old C3H mice would be a potential in vitro model of human androgenic alopecia in which the anagen phase is suppressed but the suppression is partially released by minoxidil.