This study examines the effects of an ambient concentration of aged and diluted sidestream cigarette smoke (ADSS) on bronchiolar epithelial cell development and the expression of cytochrome P450 isozyme 1A1 protein in the postnatal rat lung. In control animals, the labeling indices for epithelial cells in proximal bronchi and terminal bronchioles at 7 days of age were 2.3 and 4.9%, respectively, and decreased to 0.1 and 0%, by 100 days of age. With exposure to ADSS from birth, the labeling index of epithelial cells in distal airways of rats was significantly reduced at 7 and 14 days of age, but not in epithelial cells of proximal bronchi. The expression of P450 isozyme 1A1 antigen in bronchiolar epithelial cells of control rats reached the maximal level observed at 21 days of age and subsequently decreased to low levels at 50 and 100 days of age. In contrast, exposure to ADSS significantly increased the distribution and intensity of staining for 1A1 antigen in bronchiolar epithelial cells of proximal and distal airways as early as 7 days of age and maintained elevated levels of 1A1 protein in these cells through 100 days of age. At 21 and 50 days of age, NADPH reductase protein expression was higher in the airway epithelium of rats exposed from birth to ADSS than that noted in the airways of controls. In contrast, cytochrome P450 isozyme 2B and Clara cell secretory protein expression were unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)