1. Alpha-methyltryptophan, on administration to experimental animals gives rise to cerebral alpha-methylserotonin, which substitutes for serotonin in certain behavioral and functional tests. 2. Such results are consistent with the similarity of the properties of the two indoleamines with respect to storage, uptake and release, as well as with the 5HT2-receptor agonist activity of alpha-methylserotonin. 3. It is proposed that alpha-methyltryptophan be regarded as a candidate drug for the provision of alpha-methylserotonin as a substitute for serotonin in disorders where the latter amine is thought to be deficient.