Abstract:Alpha‐methyltryptophan (AMTP), a synthetic amino acid, is metabolized by the rat in vivo to alpha‐methylserotonin (AM5HT), which appears in the pineal gland just as it does in the brain. Pineal AM5HT assumes the same diurnal rhythm as serotonin does in control animals. Administration of AMTP results in a decrease of the serotonin content of the pineal gland, but not of its melatonin content. Pharmacological evidence indicates that the uptake of AMTP into the gland is influenced by noradrenergic innervation. No evidence was obtained for formation in vivo of alpha‐methyl‐N‐acetylserotonin or the alpha‐methyl analogue o