The O-dealkylation of ethylmorphine (EM) and codeine (CD) to morphine (M) co-segregates with debrisoquine/sparteine genetic polymorphism in man. CD O-demethylation is catalysed by cytochrome P450 2D1 (CYP2D1) in rats. In the present study, the O-deethylation of EM was examined and compared with that of CD in suspensions of freshly-isolated hepatocytes prepared by a collagenase method from Wistar rats with and without CYP2D1 inhibitors. Isolated hepatocytes were also prepared from Dark Agouti (DA) rats deficient in CYP2D1, and were incubated with EM or CD. EM, CD and their metabolites were quantified by HPLC with UV detection. EM had a similar pattern of metabolism to that of CD in suspensions of hepatocytes from Wistar rats. Both EM and CD were O-dealkylated to form M plus morphine-3-glucuronide (M3G) and N-demethylated to form norethylmorphine (NEM) or norcodeine (NCD), respectively, which were further metabolized to normorphine (NM) and finally glucuronidated to normorphine-3-glucuronide (NM3G). As compared to hepatocytes from Wistar rats, DA rats were characterized by a markedly decreased formation (70 approximately 75% reduction) of M plus M3G from both EM and CD. Quinine, quinidine, propafenone and sparteine all inhibited EM O-deethylation as well as CD O-demethylation. Quinine was the most potent inhibitor of both these O-dealkylations (Ki = 0.2 microM for both EM and CD, respectively). Quinine as well as the other inhibitors inhibited both EM and CD O-dealkylation competitively and with small differences in Ki versus EM and CD, respectively. The metabolism of EM to M plus M3G and that of CD to M plus M3G was highly correlated when results from the various separate cell suspensions were plotted. In conclusion all findings indicated that the enzyme responsible for O-demethylation of CD, CYP2D1 was also responsible for the O-deethylation of EM to M.