Epithelial-mesenchymal interactions are important regulators of lung growth and organogenesis, but later functional maturation of the fetal lung is though to be primarily under hormonal control. Recent evidence, however, suggests that hormonal effects on pulmonary cellular and biochemical maturation continue to involve intercellular interactions. Communication between cells may have wider importance than has hitherto been appreciated. It should be noted that in addition to organogenesis and biochemical development, mature lung requires an extensive capillary network in close proximity to areas of gas exchange. Recent attempts to isolate and culture pumonary endothelium may be expected to provide new insights into this neglected aspect of lung maturation. The possibility of hormonal or intercellular mediation of the differentiation of airway epithelium might also merit investigation.