The aim of the present study was to further characterize the expression of the CYP2A genes in human nasal mucosa. Fetal nasal tissues at 12-26 weeks of gestational age and surgical biopsy tissues from various regions of nasal cavity of adult patients were studied to determine whether CYP2A proteins can be detected by immunoblot in adults, whether higher levels of CYP2A proteins are present in adult than in fetal nasal mucosal microsomes, and whether CYP2A13 mRNA is more abundant than CYP2A6 mRNA in fetal nasal mucosa. In adults, immunoblot analysis detected CYP2A proteins in microsomes of the olfactory region from 8 of 10 individuals, but in none of the nasal microsomes of the respiratory region from 47 patients. Quantitative immunoblot analysis confirmed that CYP2A proteins are selectively expressed in the olfactory region in both adult and fetal tissues. Interestingly, the levels of CYP2A proteins in nasal microsomes were generally higher in fetuses than in adults. In the fetus, the level of CYP2A13 mRNA was much higher than that of CYP2A6 mRNA, as has been previously found in adult nasal mucosa. Immunohistochemical studies confirmed that, in the fetus, the CYP2A proteins are expressed in the supporting cells in the olfactory epithelium and in the Bowman's glands in the lamina propria. The prenatal expression of the CYP2A proteins in the olfactory mucosa suggests potential risks of developmental toxicity from maternally derived xenobiotics, since both CYP2A6 and CYP2A13 are known to be efficient in the metabolic activation of tobacco-specific nitrosamines and other respiratory toxicants.