Mice are particularly susceptible to carcinogens when exposure starts early in life. We evaluated the expression of stem cell antigen-1 (Sca-1) gene in the lung of variously aged CD-1 mice, either untreated or exposed to environmental cigarette smoke (ECS) and/or to a light source. Sca-1 expression progressively decreased with age. The expression of Sca-1 gene and the amount of Sca-1 protein, which was exclusively localized in endothelial cells of the pulmonary vasculature, were significantly upregulated in mice exposed either to ECS or ECS plus light throughout the weaning period, starting at birth. These findings may contribute to explain the high vulnerability of mouse lung early in life.