Pulmonary biogenic amine clearance is a carrier-mediated drug sensitive process associated with uptake into endothelial cells and subsequent metabolism by monoamine oxidase and other enzymes. Its function seems to be maintenance of arterial circulatory homeostasis by biochemically regulating circulating vasoactive hormones. Numerous pulmonary pathologic conditions are associated with alterations in these functions. However, a more precise physiologic role remains to be defined. Areas for further research include: 1. Comparative studies on the ability of endothelium from extrapulmonary and pulmonary vasculatures to extract and metabolize vasoactive amines and other humoral substances. This work must be done both in intact animals and in isolated cultured endothelium derived from organ microvasculatures. 2. Study of factors (e.g. steroids, development, etc) that may regulate pulmonary metabolic functions. 3. Measurements in humans under various clinical conditions. To this end, indicator dilution estimates and transpulmonary gradients have provided intriguing promise of means to assess endothelial metabolic functions in both normal and injured lung.