A major cause of death and illness in children under the age of five, most living in polluted cities, is respiratory disease. Previous studies have shown that neonatal animals are more susceptible to bioactivated pulmonary cytotoxicants than are adults, despite lower expression of the pulmonary CYP450's thought to be involved in bioactivation. One CYP450 that is well-documented in the bioactivation of many drugs and environmental toxicants in adult lung, but whose expression has not been evaluated during postnatal pulmonary development is CYP3A. We compared age-specific expression of CYP3A1 using 7-day old and adult male Sprague Dawley rats. Unlike what has been shown for previously studied pulmonary CYP450s, expression levels for CYP3A1 mRNA in differentiating airway cells of postnatal rats is the same as in fully differentiated airway cells of adults. CYP3A1 protein expression (28%) and enzymatic activity (23%) were lower in postnatal airways compared to adults. While other CYP450 immunoreactive proteins are primarily expressed in nonciliated cells, immunoreactive CYP3A1 protein was expressed in both ciliated and nonciliated cells in postnatal and adult rat proximal airways. CYP3A1 protein is detected diffusely throughout ciliated and nonciliated cells in 7-day old rats, whereas it is only detected in the apex of these cells in adult rats. This study demonstrates that the lungs of postnatal rats have detectable levels of CYP3A1 and that CYP3A1 mRNA expression appears not to be age-dependent whereas, steady state CYP3A1 protein levels and enzyme activity show an age-dependent manner.