Polycyclic aromatic hydrocarbons (PAHs) and metals are often environmental cocontaminants, yet there have been relatively few studies of combined effects of PAHs and metals on cytochrome P450 (P450)-catalyzed metabolism. We examined the effects of NaAsO(2) in combination with benzo[a]pyrene (BAP) on CYP1A1 and CYP1B1 in T-47D human breast cancer cells by using estrogen metabolism as a probe of their activities. Exposure to BAP caused elevated rates of the 2- and 4-hydroxylation pathways of estrogen metabolism, indicating induction of both CYP1A1, an estradiol 2-hydroxylase, and CYP1B1, an estradiol 4-hydroxylase. BAP-induced metabolism peaked 9 to 16 h after exposure and returned to near-basal levels by 48 h. Concentration-response studies showed maximal induction of the 2- and 4-hydroxylation pathways at 3 microM BAP; higher levels caused reduced rates of metabolism due to inhibition of CYP1A1 and CYP1B1. NaAsO(2) caused pronounced decreases in the induction of CYP1A1 and CYP1B1 by 3 microM BAP because cotreatment with 10 microM NaAsO(2) inhibited the rates of the 2- and 4-hydroxylation pathways by 86 and 92%, respectively. Western immunoblots showed diminished levels of BAP-induced CYP1A1 by coexposure to NaAsO(2). The levels of the CYP1A1 and CYP1B1 mRNAs induced by BAP were not significantly affected by coexposure to NaAsO(2); however, heme oxygenase 1 mRNA levels were markedly induced by coexposure to BAP and NaAsO(2). These results indicate a post-transcriptional inhibitory effect of arsenite on the expression of CYP1A1 and CYP1B1 in T-47D cells, possibly resulting from reduced heme availability.