The cytochrome P-450 monooxygenase enzymes, NADPH-reductase and form 2, were demonstrated immunohistochemically in hamster tracheal epithelium that was regenerating after mechanical injury. Bromodeoxyuridine (BrdU), a thymidine analogue, was used to map the location and extent of the wound sites between 8 and 144 h post-injury. In the control and non-wounded areas of the epithelium, the secretory cells were labelled for the monooxygenase enzymes. Label was heaviest in the apical cytoplasm of these columnar cells. At 8 h, secretory cells at the wound margins migrated to cover the wound sites, becoming progressively flattened. Reaction product for monooxygenase enzymes was strong in these flat cells but immunolabelling for BrdU was very low. At 24 h many cells at the wound sites were labelled for BrdU (indicative of a high rate of cell division). Some cells were labelled for monooxygenase but many were not stained at this time. At 48 and 72 h post-injury, none of the cells within the wound sites (regenerating epithelium) were stained. Immunochemical labelling for the monooxygenase enzymes was restored to the nascent secretory cells as they differentiated in the wound sites, beginning at 96 h post-injury. Labelling was stronger at 120 and 144 h post-injury, comparable to that in the control epithelium. The observations suggest that the monooxygenase enzymes were retained by the secretory cells in the wound sites before they divided but were lost from their progeny. Then, the temporal sequence of monooxygenase expression was similar to the pattern of differentiation of nascent secretory cells during fetal development of the tracheal epithelium.