The glucuronidation of morphine and naloxone was investigated in several regions of the human brain. Post-mortem brain tissue specimens were obtained from 19 patients 15 of whom had had cancer. With a few exceptions, all cancer patients had been treated with opiates during the terminal stage of their life. The glucuronide formation of morphine and naloxone was studied in vitro after incubation of the brain microsomal fraction with the substrate and uridine diphosphate glucuronic acid (UDPGA). The glucuronides were analyzed by high performance liquid chromatography. Glucuronidation of morphine and naloxone was catalyzed in 6 of the 19 investigated tissue specimens. The rate of formation of naloxone-3-glucuronide (N3G) exceeded that of the morphine-3-glucuronide (M3G). Morphine-6-glucuronide formation was found in only 2 specimens, in which the formation rate was 10% of the formation rate of M3G. When morphine and naloxone were present simultaneously at equal concentrations (3 mM), the N3G/M3G formation rate ratio increased compared to that when the 2 substrates were incubated one by one. Our findings are interesting from a clinical point of view since the pathways studied represent both bioactivation and inactivation steps in the metabolism of opioids.