1. During liver regeneration in the male rat, the metabolic activities of imipramine were differentially affected depending on the specific metabolic pathways. Imipramine N-demethylation was markedly reduced whereas 2-hydroxylation showed only a moderate reduction following partial hepatectomy. 2. A slight decline was observed in the hepatic microsomal content of CYP2D apoprotein, whereas a substantial decrease occurred in CYP2C11 content during liver regeneration. Since imipramine 2-hydroxylation and N-demethylation are mediated by CYP2D and 2C11 respectively, metabolic pathway-specific alterations in the activities of imipramine metabolism are explained by the isozyme selective alteration in the levels of CYPs in regenerating liver. 3. No significant effect of regeneration was observed on expression of CYP2B1 and 2E1 apoproteins. CYP3A2 apoprotein, one of the male-specific CYP isoforms, was significantly suppressed in regenerating liver showing a similar pattern of alteration to the levels of CYP2C11. The alteration pattern of the CYP1A1 level was different to the above with a moderate decline at the first day post-operation and a marked rebound thereafter. 4. In the partially hepatectomized male rate, no significant increase in androstenedione 5-alpha reductase activity, an activity predominant in the female rat, was detected. It is concluded that the pattern of alterations of hepatic oxidative metabolism during liver regeneration was not related to the functional feminization of the liver.