We developed a method for simultaneously determining naltrexone, an opioid antagonist, and its major metabolite (6-beta-naltrexol) in plasma using LC/MS/MS. Three compounds, and naloxone as an internal standard, were extracted from plasma using a mixture of methyl-tertiary-butyl ether. After drying the organic layer, the residue was reconstituted in a mobile phase (0.1% formic acid-acetonitrile:0.1% formic acid buffer, 95:5, v/v) and injected onto a reversed-phase C(18) column. The isocratic mobile phase was eluted at 0.2ml/min. The ion transitions monitored in multiple reaction-monitoring modes were m/z 342-->324, 344-->326, and 328-->310 for naltrexone, 6-beta-naltrexol, and naloxone, respectively. The coefficient of variation of the assay precision was less than 11.520%, and the accuracy exceeded 93.465%. The limit of quantification was 2ng/ml for naltrexone and 7.2ng/ml for 6-beta-naltrexol. And the limit of detection was 0.1ng/ml for naltrexone and 0.36ng/ml for 6-beta-naltrexol. This method was used to measure the plasma concentration of naltrexone and 6-beta-naltrexol in healthy subjects after a single oral 50mg dose of naltrexone. This analytical method is a simple, sensitive, and accurate way of determining the pharmacokinetic profiles of naltrexone and its metabolites. The pharmacokinetic parameters were analyzed using both non-compartmental analysis performed for each subject according to standard methods and compartmental analysis with a parent-metabolite pharmacokinetic model that was fitted to the data, simultaneously, using the program ADAPT II. The tested parent-metabolite pharmacokinetic model successfully described the relationship between the plasma concentration of naltrexone and one of its major metabolites, 6-beta-naltrexol.