Adult human hepatocytes in chemically defined culture conditions were incubated with morphine, heroin, meperidine, and methadone to investigate their potential hepatotoxicity to human liver. Cytotoxic effects were observed at about 100 times the plasma concentrations required to produce analgesia in human nonaddicts. Concentrations of 1 mM morphine, heroin, and meperidine reduced the glycogen content by 50%, while even 0.2 mM methadone produced a depletion of 70% after 24 h of treatment. Concentrations of 0.8 mM morphine and heroin, 0.4 mM meperidine, and 0.005 mM methadone inhibited the albumin synthesis by about 50% after 24 h of pretreatment. Intracellular glutathione was reduced to 50% of that of controls after 2-3 h of incubation with 2 mM morphine and 1 mM heroin, while 1 mM meperidine and 0.2 mM methadone produced a reduction of about 30% after 6 h incubation. The results show that therapeutic doses of the opioids is unlikely to produce irreversible damage to human hepatocytes, but opiate doses during tolerance or abuse may be a cause of liver dysfunction.