The effect of tryptophan hydroxylase inhibitor p-CPA and 5-HT precursor L-5HTP on conditional avoidance response and 5-HT content in rat brain was examined with the following results: Continuous administration of p-CPA improved acquisition of conditional avoidance response and retarded the loss acquired response. Body weight gain was suppressed at the low level. When the conditional avoidance response of the rats reached a certain level, it was suppressed by giving L-5HTP at the doses of 25 and 50 mg/kg in the rats given p-CPA, but no effect was observed in the rats given CMC. The 5-HT content in brain was reduced to 22% of control value by a single administration of p-CPA 316 mg/kg, but it rapidly recovered to a normal level with L-5HTP administration. From the above results, it was demonstrated that the presence or absence of p-CPA loading causes a significant difference in the appearance of behavior alteration in a rats following administration of L-5HTP.