The effect of N'-( dl-seryl)-N 2(2,3,4-trihydroxybenzyl)-hydrazine (Ro 4-4602) on the blockade in the rat of muricidal behaviour caused by 5-hydroxytryptophan (5-HTP) was investigated. It had been previously reported that Ro 4-4602 exhibited dual actions on the antagonism by DOPA of reserpine effects on motor activity. This dual effect was also observed in the present study where 5-HTP and mouse killing behaviour are employed. Lower doses (2.5–10 mg kg ) of Ro 4-4602 enhanced while the dose of 25 mg kg of Ro 4-4602 decreased the blockade of the muricidal behaviour due to 5-HTP. The biochemical interaction of Ro 4-4602 and 5-HTP in the periphery and in the brain are speculated upon in an attempt to explain the data.