To examine the role of disturbed upper airway reflexes in aspiration, we administered 20 microliters of the adenovirus (Ad) vector Ad-CMV-LacZ or 20 microliters of phosphate buffered saline (PBS) intranasally to C57 black mice. We investigated expression of the LacZ gene by this Ad vector in the nostrils of each mouse, with or without anesthesia. Under anesthesia, LacZ gene expression was detected in the lungs of every mouse given the Ad vector. However, no LacZ gene expression was found in the trachea or lungs of mice given the Ad vector without anesthesia. In mice given PBS or wild-type adenovirus transnasally during anesthesia, there was no LacZ gene expression in the nostrils, trachea, or lungs, suggesting that with 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside (X-gal) staining, blue-stained cells indicated transferred LacZ gene expression. These results suggested that aspiration of intranasal solution into lower airways was caused by disturbed upper airway reflexes during anesthesia. This process can be analyzed by the distribution of LacZ gene expression in airways. We next examined the effect of age on anesthesia-induced aspiration. Twenty-six-mo-old mice exhibited more LacZ gene expression in their lungs than did 6-mo-old mice at a concentration of 0.5 to 4.0% halothane in 100% oxygen. This suggests that light anesthesia may depress upper airway reflexes and cause aspiration in older animals. This novel model of aspiration, generated with the Ad-CMV-LacZ vector, may be useful for elucidating the mechanism of development of aspiration pneumonia in relation to age-related impairment of upper airway reflexes.