Induction of 7-ethoxyresorufin 0-deethylase activity (a cytochrome P450IA-dependent activity) by beta-naphthoflavone (0.36-mu-M) is increased 2-3-fold by dexamethasone or cortisol (10(-9)-10(-7) M) in rainbow trout hepatocyte cultures. This potentiation does not seem to be a time-dependent process and could be a classical glucocorticoid receptor-mediated event resulting in enhanced transcriptional activation of the CYP1A, as previously shown in mammals. Since glucocorticoid levels can increase in fish exposed to pollutants, such steroids may interfere with the induction response to xenobiotics.