The long term objective is to continue and complete development of a minicomputer-based interactive video image processing system for quantitative reconstructions of lung tissue and secondarily for limited three-dimensional visualizations. This will be applied to the morphometry of structures that cannot be realiably studied by conventional stereology because of their nature or because they are not randomly distributed. Moreover, the approach will also be used for confirmation in the cases where application of conventional morphometry requires excessive or problematic geometrical simplifications, or where the model and assumptions cannot be tested. The main features of our instrument are a touch sensitive screen for tracing, editing, and labelling, external fiducial marks for alignment and registration of coordinates, digital storage of compressed images, segmentation of large sections into a mosaic of small individual digital image "tiles". The instrument will be applied to five morphometric projects of significance in lung physiology: I) reconstruction of the acinus in laboratory animals, that is, the number of alveolar ducts, the individual dimensions of the ducts (length and diameter), the incremental volume and the incremental surface area, all the information needed for gas mixing studies; II) the same measurement but in human lungs from autopsy specimens; III) the study of the changes that take place during respiratory movements; this will be done by fixing rabbit lungs by vascular perfusion at 40% and 70% TLC and repeating the analysis described under I) to see what parameters changed; IV) reconstruction of endothelial capillary cells from the alveolar wall to count and study distribution of plasmalemmal vesicles; and V) re-examination of the structural parameters relevant to the sheet flow theory which permits the analysis of flow-pressure in the alveolar wall.