The effects of gonadal hormones on several parameters associated with sex-differentiated promotion in the resistant hepatocyte (RH) model were studied. Male and female rats were initiated with diethylnitrosamine and promoted with 2-acetylaminofluorene (2-AAF) and partial hepatectomy [correction of hepatecomy] (PH). Before promotion, some female rats were ovariectomized, with or without receiving subcutaneous testosterone implants. Rats were killed either at the time of cessation of 2-AAF treatment or 2 weeks later. Ovariectomy decreased the messenger RNA (mRNA) expression of the female-specific cytochrome P450 2C12 (CYP2C12) at the time of PH, but did not increase the male-specific CYP2C11. Testosterone treatment further decreased CYP2C12 and induced CYP2C11 to the level in male liver. Hepatic foci positive for the placental form of glutathione-S-transferase (GST-P) were larger in male than in female rats. Ovariectomy did not affect the size of foci, whereas testosterone treatment increased the size to the male level. At the time of cessation of 2-AAF treatment, the labeling index, determined as cells staining for proliferating cell nuclear antigen, was higher in foci of males and testosterone-treated females than in foci from females with or without ovariectomy, whereas the labeling index in the surrounding hepatocytes was lower in males and testosterone-treated females. Two weeks later, the sex differences in labeling index were still present in foci, but no differences were observed in the surrounding hepatocytes. An elevated c-myc expression was observed in nodules isolated 3 weeks after PH from males and testosterone-treated females, but not in nodules from intact females. In conclusion, ovarian hormones did not affect promotion in the RH-model, whereas testosterone administration to ovariectomized females masculinized growth hormone-regulated hepatic parameters and response to promotion. The persistent sex differentiation in focal proliferation and c-myc expression indicated that a hormone-regulated change in growth control was introduced during 2-AAF/PH treatment.