A method for quantitative measurement of delta 9-THC concentrations in plasma has been developed and applied to the analysis of more than 2000 samples over the past 4 years. The method includes addition of deuterium-labeled delta 9-THC to the plasma for use as the internal standard, a simple and relatively rapid solvent extraction procedure, formation of the trimethylsilyl derivative, and quantitation by selected ion monitoring using ammonia chemical ionization. A similar procedure has also been developed for simultaneous analysis of delta 9-THC and two of its major metabolites in plasma. The latter procedure requires use of a glass capillary column for assays of plasma samples in which the delta 9-THC concentration is below 10 ng/ml. Sensitivities for both methods permit quantitation of THC and its metabolites at concentrations as low as 0.2 ng in 1-ml plasma samples.