Smooth-muscle thickness in the distal airways of chronically challenged mice. Selected groups of mice were administered (A) dexamethasone (2 mg/kg intraperitoneally thrice weekly before each day of exposure), (B) APAS (0.1 mL undiluted, intramuscularly twice weekly), (C) busulfan (25 mg/kg intraperitoneally once fortnightly). Lungs were collected 24 hours after the last challenge in week 8. Each bar represents the mean and vertical lines the standard error of the mean. Values in parentheses indicate number of animals per group. *P < .05, **P < .01, and ***P < .001 between selected groups.