1. External counting of radioactivity over the lung has been used to measure the size of the interstitial compartment and transvascular albumin fluxes under normal and pathological conditions in the lungs of open-chested ventilated dogs.

2. When pulmonary hydrostatic pressure and plasma oncotic pressure were altered by transfusion of blood or sodium chloride solution (154 mmol/l), the transvascular transfer coefficient for serum albumin increased in relation to the increase in net intravascular force (hydrostatic pressure — oncotic pressure).

3. After alloxan, Pseudomonas bacteraemia or in the period after correction of experimentally induced haemorrhagic hypotension, the transvascular transfer coefficient for serum albumin increased to an extent greater than could be accounted for by any change in net intravascular force. This suggests that pulmonary capillary permeability to serum albumin alters with these procedures.

4. Alloxan administration caused a fulminant pulmonary oedema in which the size of the interstitial space, as measured by the external counting method, increased progressively.

5. Comparison of these results with direct studies carried out by others suggests that the external counting method provides a satisfactory technique for the study of pulmonary oedema; with simultaneous measurements of hydrostatic and oncotic pressure, it can be used to identify probable permeability changes in the pulmonary endothelium.

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