Many different types of lung models, such as mathematical, anatomic, or physiologic, have been used in experimental studies. When animal models are used, the lung is observed under the conditions of controlled, integrated cellular activity. By adding the label tritiated thymidine and using autoradiography, observed changes can be related to pulmonary cellular kinetics. These observations are a necessary prerequisite to understanding the control mechanisms of pulmonary cells. By using multiple-labeling techniques and by measuring the effects of blockade of cellular control systems, a molecular model of the lung can be developed.