In order to examine the involvement of corticotropin-releasing hormone (CRF) receptor in the formation of anxiety, we investigated whether CRF receptor antagonist CP-154,526 suppressed conditioned fear stress. First, rats were individually subjected to 30 min of footshock. Twenty-four hours after footshock, the rats were again placed in the chamber and observed for 5 min without shock. CP-154,526 was administered 30 min before placing the rats in the chamber again. After that, CP-154,526 was once more administered 30 min before applying footshock. Administration of CP-154,526 30 min both before conditioned fear stress (placing the rats inside the cage but not applying footshock) and before actual footshock significantly reduced freezing behavior. These results show that CP-154,526 blocked both the acquisition and expression of conditioned fear, thus suggesting that the CRF receptor might be related to anxiety.