1. 47 Ca kinetic studies were performed in eight normal young adult males. Plasma, urinary and faecal stable and radioactive calcium and whole-body radioactivity were measured for up to 40 days after intravenous administration of the tracer. 2. The plasma radioactivity data could be fitted to the sum of four exponentials corresponding to a four-compartment model, but this model did not account for the consistently greater 47 Ca specific radioactivities in plasma relative to those in urine at the same time in five of the subjects. 3. A non-exchangeable fraction of plasma calcium estimated as 5% of the mass in the plasma compartment was postulated to account for this finding. 4. An estimate of the insensible losses of calcium was obtained from the whole-body radioactivity counting data.