Abstract The influence of a single intraperitoneal injection or of a three-week excitatory effect of DL-5-hydroxytryptophan (5-HTP) (20–100 mg/kg IP) on lumbar motoneurons. This effect was studied by recording the integrated spontaneous electromyographic (EMG) activity of the hindlimb muscles in spinalized rats previously denervated with an intrathecal injection of 5,7-dihydroxytryptamine. A single injection of TRH 10 mg/kg IP caused by itself a stimulation of the EMG activity but failed to modify the subsequent EMG response to 5-HTP one hour or one day later. However, a three-week infusion of TRH in the lumbar subarachnoid space caused a 300% increase in the response to 5-HTP, while denervation alone caused only an increase of 160%. A similar potentiation of the effect of 5-HTP was seen on the Wet Dog Shake phenomenon which we believe is elicited in the forebrain. Therefore, chronic but not acute stimulation of lumbar motoneurons or forebrain structures by THR appears to facilitate the serotonergic excitating response. This effect of TRH does not appear to necessitate the actual coexistence of 5-HT and TRH in the same nerve fibers.