Microsomal electron transfer components cytochrome P-450, cytochrome b5, NADPH-cytochrome c (P-450) reductase activity and NADH-cytochrome c (b5) reductase activity were present in hepatic microsomes from several species of deep-sea fishes sampled between 1400 and 3200?m deep in the North Atlantic. The specific content of cytochrome P-450 varied among the species but was within the range observed for shallow-water fishes. The specific content was the highest in the deepest sampled species, Coryphaenoides armatus (0.25?nmol/mg microsomal protein). Hepatic microsomes from this species also exhibited little or no putative cytochrome P-420 whose presence suggested some degradation of cytochrome P-450 in the other species. Ethoxyresorufin O-deethylase and benzo[a]pyrene hydroxylase activities were high in several species, particularly Coryphaenoides armatus. Strong inhibition of benzo[a]pyrene hydroxylase activity by ?-naphthoflavone was demonstrated in some species. The data suggest that some hepatic cytochrome P-450 in Coryphaenoides armatus (and possibly Coryphaenoides leptolepis and Bathysaurus mollis) but not in Coryphaenoides rupestris or Antimora rostrata may have been induced by environmental chemicals in the deep sea. As in other teleost fishes, Coryphaenoides hepatic microsomes metabolized benzo[a]pyrene in vitro with a high specificity for the benzo-ring, suggesting the capacity to form mutagenic derivatives of some hydrocarbons efficiently.