The beginning of each anagen phase of the hair growth cycle appears to partially repeat the stages in the initial development of the hair cycle, but the regulatory mechanism of the hair cycle is unclear. We have investigated the levels of histamine related enzyme activities in the third hair cycle period of C3H mouse after depilation. The level of histidine decarboxylase activity increased just after depilation treatment and returned to the normal level within two weeks: this change was relevant to histamine content as we have previously reported. This result suggests that the histamine synthesising enzyme, histidine decarboxylase, activity may be involved in the distinctive process of hair re-growth, in particular, the initiation of the anagen phase.