Abstract
A simple method for establishing a constant concentration of a radioactive tracer in the blood plasma for periods of several days or weeks is described. The method consists of the subcutaneous injection of a saline suspension of a sparingly soluble compound labeled with an isotope of the element concerned. This acts as a depot of radioactivity from which slow leaching occurs into the blood stream. In the case of Ca, calcium orthophosphate labeled with / sup 45/Ca was used as the depot, and for phosphorus the same compound labeled with /sup 32/P. Some experiments with Sr were also carried out using strontium sulfate labeled with /sup 45/Sr as the depot, and the princip1e may presumably be extended to other elements of biological interest. (P.C.H.)
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