Aerobic metabolism of 9-nitroanthracene by uninduced rat liver microsomes produced four metabolites identified as trans-1,2- and 3,4-dihydrodiols, 1,2,3,4-tetrahydrotetrol of 9-nitroanthracene, and anthraquinone. Further metabolism of the predominant metabolite, 9-nitroanthracene trans-3,4-dihyrodiol, yielded a 1,2,3,4-tetrahydrotetrol with a trans-cis-trans configuration, indicating that a trans-dihydrodiol anti-epoxide is formed as an intermediate. The mutagenic activities of both dihydrodiols and 9-nitroanthracene in strains TA98 and TA100, both in the presence and in the absence of S9 enzymes, were very low. When 9-nitroanthracene was metabolized under anaerobic conditions, nitroreduction did not occur. The results thus explain the weak mutagenic activity of 9-nitroanthracene.