A gas-liquid chromatographic-mass spectrometric procedure is described for the assay of dimethyltryptamine (DMT) in whole blood. The use of a glass capillary column in combination with selective ion monitoring results in an assay with a high degree of specificity and sensitivity. 5-Methoxy-DMT is used as an internal standard and carrier in the isolation procedure. The superior resolving characteristics of the capillary column (as compared to previously employed packed columns) allows monitoring of the intense m/e 58 ion arising from the DMT side-chain. A sensitivity Emit of 10 pg/ml blood is realized with a lo-ml blood sample.