Recent reports have implied that aberrant biochemical processes in the brain are frequently accompanied by subtle shifts in the cellular epigenetic profile that might underlie the pathogenic progression of psychiatric disorders. The aim of the present study was to examine the effect of trichostatin A (TSA), a histone deacetylase (HDAC) inhibitor, on the emotional abnormality induced by maladaptation to stress in mice. Mice were exposed to repeated restraint stress for 240 min/day for 14 days. We applied dosing schedules. In one schedule, from the 3rd day of stress exposure, mice were treated with TSA (1650 μM/4 μL, i.c.v.) immediately after the daily exposure to restraint stress. In the other schedule, from the 1st day of stress exposure, mice were treated with TSA 2 h before exposure to restraint stress. After the final exposure to restraint stress, the emotionality of mice was evaluated using the hole-board test. Mice that were exposed to restraint stress for 240 min/day for 14 days showed a decrease in head-dipping behavior. This decreased emotionality observed in stress-maladaptive mice was significantly recovered by chronic treatment with TSA 2 h before daily exposure to restraint stress, which confirmed the development of stress adaptation. On the other hand, no such stress adaptation was observed under chronic treatment with TSA immediately after daily stress exposure. A biochemical study showed that tryptophan hydroxylase, the rate-limiting enzyme in serotonin (5-HT) synthesis, was increased in midbrain containing raphe nuclei obtained from stress-adapted mice that were chronically treated with TSA 2 h before daily stress exposure. These findings suggest that an HDAC inhibitor may have a beneficial effect on stress adaptation by affecting 5-HT neural function in the brain and alleviate the emotional abnormality under conditions of excessive stress.