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1. m-Xylene (1 g/kg, i.p., 1 h) increased formation of benzo(a)pyrene (BP) mutagenic bay region diols, BP-7,8-diol (66%) and BP-9,10-diol (56%) by rat pulmonary microsomal preparations, while formation of individual BP phenols and quinones was unaltered. 2. m-Xylene administration produced a decrease in cytochrome P450IIB1 activity as measured by pentoxy- and benzyloxy-resorufin O-dealkylation (PROD, BROD), while cytochrome P450IA1 activity, expressed as ethoxyresorufin O-dealkylation (EROD), was unaltered. 3. Pulmonary microsomal epoxide hydrolase activity was also unaltered by m-xylene. 4. In summary, m-xylene alters the relative contribution of P-450 isozymes to BP metabolism resulting in inhibition of BP detoxication and increased production of toxic metabolites.

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