1. The distribution of intravenously administered 18F was investigated in thirty-one normal human volunteers, seven patients with acute or chronic renal failure and two patients with malignant disease. The object was to define some features of the bodily distribution and handling of 18F in the hope that this will shed some light on fluoride physiology in health and disease.
2. Plasma radioactivity at any given time depends on the degree of diffusion of 18F through the extracellular fluid as well as on the competing effects of renal excretion and tissue uptake. The factors governing these last two functions are discussed.
3. At 1 h after injection 40% of the administered dose of 18F is contained in the extracellular fluid, 20% has been excreted and 40% taken up by the tissues, including 2·5% into erythrocytes.