1. A large-volume scintillation counter was used to measure calcium absorption from the ratio of forearm uptake of 47 Ca after oral 47 CaCl 2 (administered with milk) to forearm uptake after intravenously administered 47 CaCl 2 . 2. In some subjects serial measurements of both forearm uptake of 47 Ca and blood 47 Ca radioactivity were also recorded, and by using deconvolution both total calcium absorption and calcium absorption rate were determined. 3. The forearm ratio determination of 47 Ca absorption correlated well with that obtained by deconvolution of either serial blood 47 Ca or forearm 47 Ca measurements provided that the forearm radioactivity measurements were made at least 8 h after the administrations of 47 CaCl 2 . 4. Although the two deconvolution techniques gave similar estimates of total calcium absorption there were discrepancies between their measurements of calcium absorption rate. These discrepancies were reduced but not eliminated by the use of additional lead shielding around the Armac counter.