1. (Z)- and (E)-6-Hydroxyketamine have been synthesized and their metabolism by hepatic microsomal preparations studied to elucidate the metabolism of ketamine. 2. Both 6-hydroxyketamines are exclusively converted to 6-hydroxy-norketamines by N-demethylation. The g.l.c. retention properties and mass spectral characteristics of these 6-hydroxy-norketamines were used to confirm the structures of ketamine metabolites. 3. Ketamine is converted to norketamine, 4-, 5- and 6-hydroxynorketamines and possibly 4- and 6-hydroxyketamines in hepatic microsomal preparations from rats, rabbits and man. Norketamine is the major metabolite in all species tested. 4. 6-Hydroxynorketamine is the major hydroxylated metabolite and is found only in the (Z)-form in the species examined. 5. The metabolism of ketamine and the 6-hydroxy-ketamines is greatly increased after phenobarbital pretreatment of rats and rabbits.