A procedure for the detection and quantitation of 11-nor-delta 9-tetrahydrocannabinol-9-carboxylic acid, the major metabolite of delta 9-tetrahydrocannabinol, in urine is presented. Because a significant portion of the metabolite is present as a conjugated form, the urine was hydrolyzed by the addition of strong base. The solution was then acidified and the metabolite extracted into an organic solvent. It was subsequently converted to the t-butyldimethylsilyl ether and t-butyldimethylsilyl ester, and analyzed by GC/MS utilizing electron ionization (EI). Confirmation of the product was carried out by using selected ion monitoring (SIM) for three ions which represent logical demonstrable fragmentation pathways for the molecule and by comparing their relative abundances to a reference standard. A deuterated analog was carried through the entire process as an internal standard. The method provides excellent linearity and the derivatives are stable for more than 10 days at room temperature.