The purpose of the present study was to investigate the role of specific CYPs responsible for the O-dealkylation of ethylmorphine (EM) and codeine (CD) to morphine (M), as well as that of norethylmorphine (NEM) and norcodeine (NCD) to normorphine (NM) in rat liver microsomes. Liver microsomes metabolize EM and CD to M, and NEM and NCD to NM, in the presence of an NADPH-generating system. The metabolites of EM and CD were determined by HPLC with UV and electrochemical detection. In the present study, the role of CYP2D1 in O-dealkylation of EM/NEM and CD/NCD was investigated by use of specific antiCYP antibodies. When testing rabbit antirat CYP2D1, 2E1, 2C11, and 3A2 antibodies, only the antiCYP2D1 antibody inhibited the EM/NEM and CD/NCD O-dealkylase activities significantly. The maximum inhibition achieved was approximately 80% at a protein ratio (IgG to microsomes) of 10:1, p = 0.001. The contribution of CYP2D1 to the O-dealkylation of EM/NEM and CD/NCD was further confirmed by use of the specific CYP2D1 inhibitors quinine and propafenone. Five microM of quinine inhibited the EM/NEM and CD/NCD O-dealkylase activities by approximately 80%. The CYP3A inhibitor troleandomycin (TAO) failed to inhibit the CYP2D1 catalyzed reaction, but did inhibit the N-demethylation of EM and CD. The O-dealkylation of NEM and NCD was also impaired in Dark Agouti rat (DA) liver microsomes. Taken together, the immunoinhibition and chemical-inhibitor studies of rat liver microsomes provided convincing evidence for the involvement of CYP2D1, the rat counterpart of human CYP2D6, in the metabolism of EM/NEM and CD/NCD to the corresponding O-dealkylated metabolites.