A rapid quantitative method was developed to assay in urine naltrexone and its major urinary metabolite, beta-naltrexol. Following solvent extraction and elimination of interfering materials, the weakly basic drug, its metabolite and the internal standard (etorphine) were silylated and analyzed by gas-liquid chromatography. As little as 0.02 mug/ml of naltrexone and beta-naltrexol was detectable using a hydrogen flame ionization detector. Twenty-four-hour urine samples were analyzed from three subjects taking 180 mg naltrexone daily. The major urinary excretion product was beta-naltrexol which accounted for 48.6% of the administered dose. Only 5% of the administered dose excreted in the urine was naltrexone. Beta-naltrexol was excreted 70% as free drug and 30% conjugated while naltrexone was 90% conjugated.