Rat livers display a sex-specific cytochrome P450 (P450) isoforms expression pattern with consecutive differences in P450-mediated monooxygenase activities, which have been shown to be due to a differential profile of growth hormone (GH) secretion. Parallel to previous investigations on P450 isoforms expression, the aim of the present study was to elucidate the influence of recipient gender on P450-mediated monooxygenase activities in intrasplenic liver tissue transplants in comparison to orthotopic liver. Fetal liver tissue suspensions of mixed gender were transplanted into the spleen of adult male or female syngenic recipients. Four months after grafting transplant-recipients and age-matched controls were treated with beta-naphthoflavone (BNF), phenobarbital (PB), dexamethasone (DEX) or the vehicles and sacrificed 24 or 48 h thereafter. P450-dependent monooxygenase activities were assessed by a series of model reactions for different P450 subtypes in liver and spleen 9000 g supernatants. In spleens of male and female control rats only very low monooxygenase activities were detectable, whereas with most model reactions distinct activities were observed in transplant-containing organs. Livers and transplant-containing spleens from male rats displayed higher basal ethoxycoumarin O-deethylase and testosterone 2alpha-, 2beta-, 6beta-, 14alpha-, 15alpha-, 15beta-, 16alpha-, 16beta- and 17-hydroxylase activities than those from females. On the other hand, like the respective livers, spleens from female transplant-recipients demonstrated more pronounced p-nitrophenol- and testosterone 6alpha- and 7alpha-hydroxylase activities than those from male hosts. With nearly all model reactions gender-specific differences in inducibility by BNF, PB or DEX could be demonstrated in livers as well as in transplant-containing spleens. These results further confirm that the P450 system of intrasplenic liver tissue transplants and the respective orthotopic livers is similarly influenced by recipient gender.