The expression level of phospholipase D (PLD) was examined in the lungs of mice during postnatal development. Immunoprecipitation analysis revealed a moderate basal level of PLD1 in the lungs at postnatal 1 week, with a gradual increase occurring until week 4. The pattern of PLD2 paralleled that of PLD1, although the quantity of PLD2 was markedly less. PLD was immunostained in the most bronchiolar epithelium, some pulmonary macrophages and alveolar cells at postnatal week 1, with increased immunoreactivity in those cells with lung development at postnatal week 4. These findings suggest that the increase of PLD expression is associated with pulmonary alveolarization, possibly by involvement in cell proliferation and differentiation during postnatal mouse lung development.