Summary The washout from the liver of xenon-133 injected into the thoracic aorta of human subjects undergoing cardiac investigation was monitored externally with a sodium iodide scintillation detector. The washout curve displayed an initial peak of activity, followed by a brief decline, and then a second, slower rise. The initial peak was thought to be due to isotope arriving in the liver via the hepatic artery, and the second rise due to isotope arriving via the portal vein. A method of analysis is presented which uses the height of this arterial peak to calculate arterial perfusion of the liver.