The effects of the topical application of beclomethasone dipropionate (a corticosteroid) on the aryl hydrocarbon hydroxylase (AHH) activity in epithelial tissue (lung and skin) were studied. To evaluate the results of the pre-treatment with beclomethasone dipropionate a comparison was made with the effects of the topical application of benzanthracene (a polycyclic aromatic hydrocarbon and a known inducer of AHH activity). AHH activities were increased 2.5 to 5-fold in lung and about 20-fold in skin of mice after topical application of benzanthracene. However, after pre-treatment with beclomethasone dipropionate no induction of AHH activity could be observed, either in lung or in skin. The implications of these findings are discussed.