1. A method for measuring the pulmonary water compartments in vivo in anaesthetized dogs is described.

2. The technique depends upon the sequential intravenous injection of markers labelled with γ-ray-emitting isotopes. Each is allowed to become evenly distributed throughout the compartment to which it is confined. A calibrated γ-counter placed at the surface of the inflated lung then records a count rate related to the total quantity of isotope in unit volume of tissue. The size of each water compartment can be calculated from the external count rate and the isotopic concentration of the marker in plasma.

3. The markers used were 125I-labelled serum albumin for plasma, [125I]iodide for extracellular water and [125I]iodoantipyrine for total water.

4. The method was extended to calculate transcapillary flux rates of serum albumin and showed that rapid exchange of serum albumin takes place between the plasma and interstitial space.

5. Results obtained non-invasively by the isotopic method were compared with direct measurements. Agreement was good.

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