Abstract: The possibility of involvement of cytochrome P4503A(CYP3A) in the monohydroxylation of the ring A of praziquantel(PZQ) was studied by using CYP3A specific inducer dexamethasone(DEX), specific inhibitor triacetyloeandomycin(TAO) and the activity of erythromycin(ERY) and ethylmorphine(EMP) N-demethylase which are known to be marker for CYP3A enzyme activity as probes. In the liver microsomes obtained from rats pretreated with CYP3A inducer DEX with TAO treatment the content of uncomplexed P450 was decreased, the activity of ERY and EMP N-demethylase was reduced, and simultaneously, the rate of formation of ring A monohydroxylate of PZQ was inhibited. Ring A monohydroxylate formation was competitively inhibited by TAO and ERY. The rates of ring A monohydroxylate formation were strongly correlated with the activity of N-demethylase of ERY and EMP. These results indicate that CYP3A is involved in the monohydroxylation of the ring A of PZQ.