Species differences in the interactions of cyclosporine A (CSA) and tacrolimus (TAC) with the cytochrome P450 (CYP) system in male rat and human liver were investigated in vitro by assessing effects on a series of model reactions for different CYP isoforms. CSA and TAC concentration dependently inhibited ethoxyresorufin O-deethylation, ethoxycoumarin O-deethylation and pentoxyresorufin O-depentylation and 7alpha- and 17-testosterone hydroxylation (TH) activities in both species. In rat liver no effect of CSA was seen on ethylmorphine N-demethylation and 2alpha- and 6beta-TH activities, but an inhibition due to TAC. Both CSA and TAC, however, distinctly decreased ethylmorphine N-demethylation and 2beta- and 6beta-TH activities in human liver. The same results were seen with 14alpha- and 15beta-TH activities. 2alpha-, 16alpha- and 16beta-TH activities were only inhibited in human liver with TAC, whereas only in this case 6alpha-TH activity was left unaffected. p-Nitrophenol hydroxylase activity was not influenced by either substance in both species. Thus, CSA mainly interacts in rat with the CYP isoforms 1A, 2A and 2B and in man with the CYP subtypes 1A, 2A, 2B, 2C and 3A. TAC seems to interfere predominantly in rat with the CYP isoforms 2A, 2B, 2C and 3A and in man with the CYP subtypes 1A, 2B, 2C and 3A. In summary, our results point to distinct species differences in the interactions with the CYP system with both substances, and although from literature CSA and TAC are known to be metabolized mainly by CYP 3A, according to our findings in rat liver CSA seems not to interact with this CYP subtype.