The activity of aryl hydrocarbon hydroxylase (substrate: benzo(a)pyrene) was increased in the tracheas of rats exposed to cigarette smoke for 1 h daily for either 1 or 10 days. However the degree of increase in activity was lower in the trachea than in the lung. After a single exposure, activity in the trachea was at its highest level 12 h following exposure (3.2-fold compared to the control), but had returned to the control level within 24 h. Also, the amounts of covalently bound metabolites of benzo(a)pyrene were increased (2-fold) in nucleic acid and protein fractions of the trachea, when the rats were killed 12 h after a single cirgarette smoke exposure. No significant changes in activity of epoxide hydratase (substrate: styrene oxide) could be detected in the trachea. After repeated exposures the activity of UDP-glucuronosyltransferase (substrate: methylumbelliferone) was increased (1.7-fold) in the trachea.