Abstract: The level of 5-hydroxytryptophamine(5-HT)and 5-hydroxyindole acetic acid (5-HIAA) in rat brain was significantly incroased during the anaesthetic period induced by scopolamine intracerebroventricularly. The duration of anaesthesia by scopolamine in mice was markedly prolonged as the result of the loading dose of L—tryptophan (400mg/kg ip) . However, the anaesthetic effect was not abolished despite the depletion of 5-HT and 5-HIAA in rat brain after p—chlorophenylalanine. It was also noticed that the level of dopamine in midbrain was raised, but no significant change of noradrenaline level was observed. Potentiation of 5—HT system in rat brain could synergise the anaesthetic effect of scopolamine but it did not seem that the functional normality of 5—HT system in the brain was requisite to create this kind of anaesthesia. It was also suggested that the transmitting function of tryptophanergic neurones in the brain might be accelerated by M-cholinergic receptor.