OBJECTIVE To study the expression of inducible nitric oxide synthase (iNOS) in livers of mice infected with Schistosoma japonicum. METHODS The livers of NMRI mice infected with S. japonicum were collected on day 21, 28, 38, 45 post infection (p.i.), total RNA of livers were extracted and kinetics of the mRNA expression of iNOS were detected by RT-PCR, the protein expression of iNOS was then confirmed by Western blotting and the distribution of iNOS in the infected liver was determined by immunohistochemical methods. RESULTS The mRNA expression of iNOS was not detectable in the uninfected liver, iNOS mRNA expression was detected on day 21 p.i., the expression increased on day 28 p.i. and peaked on day 38 p.i., then decreased slightly on day 45 p.i. Western blotting showed an iNOS expression in the livers only on day 38, 45 p.i. IFA test showed that the expression of iNOS was mainly distributed in the granuloma of the livers. CONCLUSION S. japonicum infection can induce the expression of iNOS in a time-dependent manner in the liver of the host, and eggs may be the main factor in inducing the expression.