To determine whether there might be a physiologic basis for the abuse of ethanol by methadone maintenance patients, studies of methadone metabolism were conducted in vivo and in vitro after acute and chronic ethanol administration to rats. Acute administration of ethanol resulted in increased brain and liver concentrations of methadone at two time periods and decreased biliary output of pharmacologically active methadone measured by radioactive tracer technique with [3H]methadone. In vitro, ethanol inhibited N-demethylation of methadone by microsomes from livers of naive rats, with a Ki averaging 32 mM (146 mg/kl). Also, methadone metabolism was inhibited by carbon monoxide indicating cytochrome P-450 dependence as is the case for ethanol. Chronic administration of ethanol to rats led to decreased levels of unmetabolized methadone in brain and liver, in vivo, and to increased microsomal N-demethylation of methadone, in vitro, compared to pair-fed controls. Therefore, acute ethanol administration may enhance cerebral effects of methadone by interfering with degradation of methadone at liver microsomal sites. Chronic ethanol administration increases methadone metabolism by stimulating degradation in liver microsomes.