A procedure for the quantitative measurement of tryptamine in mammalian tissues is described. The amine is isolated by ion-exchange chromatography, converted to its dansyl derivative, further purified by thin-layer chromatography, and quantitated by the mass-spectrometric integrated ion current technique using an isotopically labelled internal standard.The concentrations of tryptamine in some tissues of male Wistar rats were (ng/g ± S.D.): brain 0.50 ± 0.07, heart 0.62 ± 0.10, kidney 8.04 ± 2.10, liver 0.73 ± 0.07, lung 0.54 ± 0.18, and spleen 0.43 ± 0.14. In the brain, the hypothalamus contained 0.94 ± 0.22, the cerebellum 0.27 ± 0.02, the stem 0.24 ± 0.06, the caudate nucleus 2.93 ± 1.14, and the "rest" 0.32 ± 0.05 ng/g (mean ± mean deviation).