The N,N-dimethylated metabolites of tryptamine and serotonin, dimethyltryptamine (DMT) and bufotenine, have been identified in human plasma and urine using a variety of analytical techniques including gas chromatographymass spectrometry (CC-MS) [l-4]. An enzyme activity capable of forming the dimethylated compounds from the endogeneous non-methylated substrates has been demonstrated in several mammalian tissues [ 5. -71. The physiological significance of the N-methylating pathway of indokarrine metabolism and of the methylated end products remains unknown at present. Because of the known psychotropic properties of the dimethylated amines, their possible involvement in the chemical pathogenesis of mental disorders has received -wide interest [S-l1 J _ Comparative studies have been conducted on the activity of the N-methylating enzyme and on both the blood levels and the urinary excretion of the dimethylated amines in normal subjects and mentally disturbed patients. Somewhat contradictory findings have been reported. The dimethylated amines have been detected more frequently in the urine of psychotic patients than in the urine of normal controls 12, 12, 131. Also, an elevated activity of the N-methylating enzyme has been claimed as present in the plasma of schizophrenic patients 1143. In other studies, however, no significant differences in either the urinary excretion or the blood levels of the dimethylated metabolites, or in the N-methylating enzyme activity have been detected [4,8,9,15] _ Several of these studies are based on the qualitative identification or a semiquantitative estimation of the dimethylated metabolites in the extracts of urine and plasma [l, 2,9,13]. A mass fragmentographic (MF) isotope dilution assay for DMT has been presented and applied to clinical studies [4,15-171. Also gas chromatography with nitrogen detection has been used for the quantification of DMT [18]. Although mass spectrometry (MS) has been used for the identification of bufotenine in the urine ]2] , no quantitative MF technique has been so far pr&sented. _ In this paper we describe a quantitative isotope dilution assay for DMT and bufotenine based on MF. Both compounds can be assayed from the same sample as trimethylsilyl (TMS) derivatives. The deuterated analogues of the two compounds have been synthesized for use as carrier substances and internal standards. The isolation procedure for the dimethylated amines has been modified by introduc’fig the non-ionic XAD adsorbent resin for extraction of the amines from aqueous solution. This facilitates the handling of large urine samples and applied together with a thin-layer chromatographic step reduces the contamination of the GC-MS instrument.