A GLC-mass spectral analysis with a deuterated internal standard was developed to measure plasma mescaline concentrations after intravenous administration to rabbits. The drug and the internal standard were extracted with benzene, derivatized with trifluoroacetic acid anhydride, and chromatographed on 2.5% QF-1 with mass fragmentographic detection. The detection limit is 5 ng/ml of plasma. The relative standard deviation was approximately 5%. The main advantage of this method is that it combines the specificity of the GLC retention time and mass spectral fragmentation pattern with the sensitivity of the mass fragmentographic detection.