1. Pretreatment of female rats with (−)-emetine or (±)-2,3-dehydroemetine (at 18μmol/kg body wt. for 24h) prolongs the hexobarbital-induced sleeping-time of the treated animals. 2. This effect is not observed on pretreating animals with other compounds closely related to (−)-emetine, such as (−)-isoemetine or (+)- O -methylpsychotrine. 3. Liver microsomal drug-metabolizing enzyme activity in vitro as measured by N -demethylation of aminopyrine and azo-reduction of Neoprontosil is inhibited in rats pretreated with (−)-emetine or with (±)-2,3-dehydroemetine. 4. These inhibitory effects on drug metabolism in vitro are not observed in corresponding experiments involving pretreatment of rats with (−)-isoemetine or (+)- O -methylpsychotrine. 5. Co-administration of emetine or 2,3-dehydroemetine and sodium phenobarbital or 1,1-dichloro-2- o -chlorophenyl-2- p -chlorophenylethane to rats abolishes or greatly diminishes the stimulation of drug-metabolizing enzyme activity in vitro usually obtained by the administration of phenobarbital or 1,1-dichloro-2- o -chlorophenyl-2- p -chlorophenylethane alone. 6. Further, in rats pretreated with sodium phenobarbital and subsequently injected with emetine or 2,3-dehydroemetine the pre-stimulated drug-metabolizing enzyme activity in vitro is diminished. 7. The inhibitory effects on drug-metabolizing enzyme activity after pretreatment with (−)-emetine or (±)-2,3-dehydroemetine do not appear to be related to NADPH generation.