1. Different radioisotopes of calcium were administered orally and intravenously to a series of patients, some normal and others with diseases that might affect calcium absorption or metabolism. Concentrations of the two isotopes in plasma were determined at intervals. A mathematical procedure was developed for deriving from these measurements the total absorption of the oral dose and also the absorption/time relationship. 2. The correlation between absorption and plasma concentration of the oral dose at various times after administration was studied and found to be very good, the 2 hr plasma level giving the best correlation, particularly when account was taken of the patient's weight. 3. The effect of variations in the disappearance curve of the injected isotope upon the shape of the plasma appearance curve of the oral isotope was examined. It is concluded that for the majority of cases, when absorption is not greatly delayed, and the exchange processes determining the shape of the disappearance curve are not markedly abnormal, 2 hr plasma concentration gives a good indication of absorption. When abnormalities in these processes are present, only the double isotope method gives accurate results. 4. The results were applied to examine the validity of other published methods of analysis. It was found that plasma concentrations of orally administered radiocalcium alone can give useful information about absorption, but not about plasma clearance.