Marijuana, which is made from crushing the leaves, flowers, and sometimes the stems of the plant Cannabis sativa, contains more than 30 cannabinoids. The major psychoactive cannabinoid is delta-9-tetrahydrocannabinol (THC). The major metabolite of THC, 11-nor-delta 9-carboxy-tetrahydrocannabionol (THC-COOH), is excreted in the urine primarily as a glucuronide conjugate and is commonly analyzed in biological specimens for detecting marijuana usage. The procedure described here involves the addition of deuterated internal standard THC-COOH-d9 into the sample followed by hydrolysis of conjugated THC-COOH by alkali. THC-COOH is extracted from urine or blood using liquid-liquid extraction followed by preparation of its trimethylsilyl derivatives. The analysis of derivatized THC-COOH is performed using gas-chromatography/mass spectrometry (GC/MS). Quantification of the drug in a sample is achieved by comparing the responses of the unknown sample to the responses of the calibrators using selected ion monitoring.