A specific high-pressure liquid chromatographic assay was developed for the measurement of the narcotic antagonist drug, naltrexone. When 10 mg of naltrexone . HCl was added to the perfusate of the isolated perfused rat liver, the concentration of the drug in the perfusate declined rapidly with an apparent half-life of about 9 min. At 30 min 90% of the unchanged drug had been removed from the perfusate; the liver at this time retained 47% of the total radioactivity, of which only 75 represented unchanged drug. The bile at 30 min contained 28% of the total radioactivity and by 120 min this had risen to 70%. Initially naltrexone glucuronide was the principal metabolite in the bile. After 60 min N-dealkylated naltrexone glucuronide was the principal metabolite in bile. A marked increase in bile flow was noted immediately after the addition of the drug to the perfusate. The choleretic effect was attributed to the osmotic effect of the metabolites of naltrexone in the bile.