There has been no noninvasive, readily available method for clinically evaluating changes in pulmonary metabolism of biogenic aminelike substances. Such metabolic changes, which almost surely take place on the surface or within the endothelial cells that line the pulmonary vasculature, are likely to be significant components of the overall clinical response of the lung to many disorders. Radioiodinated metaiodobenzyl-guanidine (MIBG) shows properties in isolated, perfused lung preparations that simulate those of biogenic amines (eg, its uptake is sodium-dependent and virtually abolished by ouabain). New data from studies in sheep and humans indicate that changes in first transit pulmonary extraction of this compound can easily be monitored externally using conventional gamma camera-computer systems. MIBG labeled with iodine 123 may provide an important new means for clinical assessment of changes in pulmonary endothelial metabolism.