Abstract: In this paper, the effect of Schistosoma japonicum infection on liver drug-metabolizing enzymes of mice was studied. The prolongation of hypnosis duration of sodium pentobarbital in vivo and the inhibition of liver aniline hydroxylase (AH), aminopyrine N-demethylase (APD) as well as cytochrome P-450(Cyt P-450) in vitro were observed in mice infected with 20,40 and 60 cercariae of S. japonicum after 6 weeks. Meanwhile, with 30 cercariae infection, no significant inhibition of the activity of AH, APD and Cyt P-450 was observed in mice 4 weeks after infection, but significant inhibition of these enzymes was shown by 8 weeks . As compared with control , the activity of AH , APD and Cyt P - 450 decreased to 32. 6%, 13. 7% and 7. 7% at 8 weeks. The results indicate that infection of S. japonicum can inhibit the liver drug-metabolizing capacity of mice, and the degree of inhibition depands on the infectivity and the duration of infection.