The effects of acute administration of thyrotropin-releasing hormone (TRH) on the fear motivated learning of male Wistar rats were studied using a shuttle box. Two kinds of experiments - active avoidance learning (shuttle avoidance learning) and passive avoidance learning were performed. TRH (500 microgram/kg) was injected subcutaneously 40 to 50 minutes before the test trials. 1. In shuttle avoidance learning, TRH facilitated the acquisition after 8 sessions-pretraining and retarded the extinction after 8 approximately 12 sessions-pretrainings compared with saline. In passive avoidance learning, however, the reverse results were obtained: TRH retarded the acquisition and facilitated the extinction. 2. In shuttle avoidance learning, the acquisition facilitating effect of TRH was seen only in poor performer rats (poor performer was defined as less than 37 median score of conditioned avoidance responses in 8 sessions-pretraining), but not in good performer rats (more than 37 conditioned avoidance responses). 3. In shuttle avoidance learning, the acquisition facilitating effect of TRH was not seen in poor performer rats after 15 sessions-pretrainings, and the extinction retarding effect of TRH was not seen in the rats after 28 sessions-pretrainings. 4. From the above results it can be concluded that TRH possesses a fear reducing effect, and the behavioral changes brought about by TRH are seen most obviously in the rats which are in highly fearful conditions.