1-Nitropyrene, a tumorigenic environmental pollutant, is mutagenic in Chinese hamster ovary (CHO) cells in the presence of a liver homogenate 9000 g supernatant fraction (S9). The metabolic pathways involved in this response were studied by comparing the mutagenicities at the hypoxanthine-guanine phosphoribosyl transferase locus of 1-nitropyrene, some oxidized microsomal metabolites of 1-nitropyrene, and related compounds. In the absence of S9, pyrene 4,5-oxide and 6-hydroxy-1-nitropyrene displayed the highest mutagenicities, followed by 1-nitropyrene 9,10-oxide and 1-nitropyrene 4,5-oxide; 3- and 8-hydroxy-1-nitropyrene were weaker mutagens, while pyrene and 1-nitropyrene were essentially without activity. With S9, the order of mutagenic potency was 1-nitropyrene 4,5-oxide greater than 6-hydroxy-1-nitropyrene approximately 1-nitropyrene 9,10-oxide greater than 1-nitropyrene approximately 3-hydroxy-1-nitropyrene approximately 8-hydroxy-1-nitropyrene greater than pyrene approximately pyrene 4,5-oxide, with the latter two compounds being essentially inactive. Inclusion of the epoxide hydrolase inhibitor 1,2-epoxy-3,3,3-trichloropropane during the S9-mediated treatment of CHO cells with 1-nitropyrene increased mutation induction 5-fold. Also, liver microsomes prepared from guinea-pigs treated with Aroclor 1254 mediated a stronger mutagenic response with 1-nitropyrene than microsomes from Aroclor-treated rats. 1-Nitropyrene was essentially non-mutagenic in the presence of microsomes from untreated and phenobarbital-treated rats. Examination of the 1-nitropyrene metabolites produced during the microsomal incubations indicated that Aroclor-induced guinea-pig microsomes yielded substantial amounts of 1-nitropyrene 4,5-dihydrodiol, while Aroclor-induced rat microsomes produced 6-fold more 6- and 8-hydroxy-1-nitropyrene than phenobarbital microsomes.(ABSTRACT TRUNCATED AT 250 WORDS)