A procedure for the quantitative evaluation of m-tyramine in mammalian tissues is described. It involves isolation of the amine by ion-exchange chromatography, followed by conversion to the dansyl derivative, chromatographic separation, and quantitation by the mass spectrometric integrated ion current technique using an isotopically labelled internal standard. The concentrations of m-tyramine in some tissues of male Wistar rats were (mean ± S.D., nanograms per gram): brain 0.32 ± 0.03, heart 0.44 ± 0.13. kidney 12.6 ± 3.4, liver 0.27 ± 0.04, lung 0.33 ± 0.11, spleen 0.25 ± 0.07, and blood 0.15 ± 0.04.