Olfaction is a crucial function in most fish species, but little is known about biotransformation enzymes in the olfactory organ. This study demonstrates that biotransformation enzymes usually found in the rainbow trout liver, are present in the olfactory organ as well. While microsomal cytochrome P450 reductase, p-nitrophenol hydroxylase and cytosolic glutathione S-transferase presented similar levels in both the olfactory organ and the liver, microsomal 7-ethoxyresorufin O-deethylase (EROD), 7-ethoxycoumarin O-deethylase, and 7-pentoxyresorufin O-deethylase were much lower in the olfactory organ (77-, 35-, 200-times respectively). Furthermore, microsomes from the olfactory organ were able to perform testosterone hydroxylation only in the 16 alpha-position while testosterone was hydroxylated in the 16 beta-position by liver microsomes. Using polyclonal antibodies raised against perch cytochrome P4501A1, the immunoreactive protein was shown to be strongly expressed in various cellular types forming the nonsensory epithelium. Some immunostaining was also reported in the nonsensory cellular elements constituting the sensory epithelium, while olfactory receptor cells failed to show cytochrome P4501A1 -immunoreactivity. Finally, the exposure of rainbow trout to waterborne beta-naphthoflavone (0.1 mu g ml(-1)) for 2 or 4 days resulted in a higher induction of EROD activity in the olfactory organ compared to the liver. The presence of biotransformation enzymes in the olfactory organ of rainbow trout addresses the question of their involvement in the detoxication/toxication of pollutants as well as in the olfactory function.