NADPH-, NADH- and cumene hydroperoxide-dependent metabolism of benzo[a]pyrene by pyloric caeca microsomes of the sea star Asterias rubens L. (Echinodermata: Asteroidea). 1. Benzo[a]pyrene (BaP) metabolism was studied in microsomes of the pyloric caeca (main digestive tissue and site of P450) of the echinoderm sea star (starfish) Asterias rubens. 2. NADPH-dependent metabolism of BaP produced phenols (36% of total metabolism), quinones (19%), dihydrodiols (25%) and putative protein adducts (20%). 3. NADH-dependent rates of BaP metabolism were approximately twice those found for NADPH-dependent metabolism, and metabolite formation was shifted towards dihydrodiols and quinones. 4. Cumene hydroperoxide (CHP)-dependent rates of BaP metabolism were also higher than NADPH-dependent rates by a factor of six for quinone and putative protein adduct production, and by a factor of four for phenol and dihydrodiol production. 5. Microsomal rates of BaP metabolism in BaP-exposed sea stars appeared to be elevated more in the case of NADPH-dependent than for CHP-dependent metabolism (respectively, increases of 130 and 41%), indicating the induction of forms of P450 preferentially catalysing NADPH-dependent metabolism. 6. 1,1,1-Trichloropropene-2,3-oxide (TCPO) inhibited dihydrodiol formation from both NADPH- and CHP-dependent BaP metabolism, indicating the involvement of epoxide hydratase in BaP metabolism. 7. Incubations of pyloric caeca microsomes with BaP and a superoxide anion radical-generating system (xanthine/xanthine oxidase) produced putative protein adducts but no free metabolites.