The ultrastructure of lung capillaries and of lung interstitiuni has been studied in isolated. ventilated and plasma-perfused rabbit lung preparations at different levels of perfusate [Ca++] and [Mg++] (achieved by EDTA additions). A perfusion fixation niethod was applied, and situations where net fluid transfer over the capillary wall was taking place could therefore be studied with some confidence. No morphological changes were found which could explain the increased capillary permeability to proteins during EDTA-induced edema development. Ultra-structural alterations of the capillary walls were similarly not observed during periods of increased hydraulic conductivity of the lung capillaries caused by lower concentrations of EDTA than those releasing edema. In all experiments where net outward flux of fluid from the capillaries had taken place at the moment of fixation, one could observe electron lucent zones (laminae rarae) of the basement membranes which were much more distinct than in control specimens. This phenomenon could be related to the process of capillary fluid leakage, which thus may take place also in the regions of the vessel wall wheie the alveolar-capillary barrier is at its thinnest.