We demonstrated emotional piloerection in mice given conditioned fear stress by means of a pass-through apparatus. The emotional piloerection was first assessed in mice of different ages. The results showed that the piloerection changed with age. Pharmacological studies showed that the piloerection was inhibited by an alpha 1-adrenoceptor antagonist prazosin, but, surprisingly, was not inhibited by anxiolytic diazepam. These findings strongly suggest that the neuronal system of piloerection is different from that of freezing behavior, and that the neuronal system of piloerection develops with age.