Administration to rats of alpha-methyltryptophan (AMTP) gives rise to alpha-(AM5HT) in the brain along with a decrease of cerebral 5HT. 2. Analysis of fractions prepared from brains of AMTP-injected rats shows that AM5HT occurs mainly in the synaptosomes. 3. The synaptosomal content of AM5HT in proportion to the total AM5HT in the brain represents the same ratio as for the corresponding fractions of 5HT.