Whereas the respiratory function of the lung has been studied extensively, there are only scarce data available concerning the lung's drug clearance capabilities in man. Its metabolic function in hormonally active agents has been documented in animals. To gain insight in this non-respiratory function of the lung knowledge of the architecture of the alveolar-capillary unit and the histochemistry of its different cell types is necessary. Some examples of studies with drugs are presented to illustrate the methods that have been used in metabolic and uptake studies of the lung.