A method was developed for the continuous infusion of 15O-labeled water which allows the tomographic reconstruction of the total lung water (TLW). Subsequent inhalation of 11C-labeled carbon-monoxide permits the reconstruction of the blood volume (BV). After normalization of intravascular activities the difference of TLW minus BV yields a quantitative value of regional extravascular lung water (rELW). 15O-O2 is converted on-line to 15O-H2O and trapped in a 2 ml buffer reservoir which is fed by a pump with 0.9% NaCl. A precision pump is used to withdraw the labeled H2O and infuse it at a rate of 6 ml/min. The radioactivity level of the infusate (ca. 3.7 MBq/sec) is controlled and can be kept constant with a deviation of less than 5% over 40 min. The sterility and apyrogenicity of the system effluent is assured by frequent bacteriological, rabbit and limulus tests. A constant radioactivity level in the lung area is reached after 8-10 min. The infusion is continued for the tomographic reconstruction (Positron Camera System 4200, Cyclotron Corp.) which takes 15 min. A fast change of cyclotron parameters (MC-36, Scanditronix) and automated chemistry procedures allow a single breath administration of 11C-CO (ca. 40 MBq) 15 min after the end of the 15O-H2O infusion. Blood pool equilibrium is reached after 3-4 min, and the blood volume is reconstructed within 15 min also. Intravascular activities as determined from reconstructed slices in the region of the aortic arch correlate linearly with blood sample activities up to 100 kBq/ml.