Abstract—Rats were injected intracerebrally with labelled tryptamine, 5‐hydroxytrypt‐amine (5‐HT) and norepinephrine (NE). The disappearance of the amines and their metabolites as a function of time was determined. Tryptamine disappeared very rapidly, with a half‐life of 5 min in normal animals and of 45 min in rats treated with a monoamine oxidase (MAO) inhibitor. The level of radioactive 5‐HT declined in two phases, with half‐lives of 45 min and 3 h respectively. The 5‐hydroxyindoleacetic acid (5‐HIAA) that was formed disappeared with a half‐life of approximately 1 h. After inhibition of monoamine oxidase, there was only a single phase of 5‐HT disappearance (half‐life of 4 h). Reserpine decreased and imipramine increased the amount of 5‐HT remaining 4 h after injection. Of the NE injected, 12 per cent was converted to methoxyhydroxyphenylglycol sulphate (MHPGS), which disappeared with a half life of 3 h. Reserpine doubled the amount of methoxyhydroxyphenylglycol sulfate formed, but did not alter its rate of disappearance, its peak concentration occurring about 30 min after injection of NE in both control a