1. Administration of a large amount of unlabelled calcium orally 60 min after intravenous injection of 47 Ca leads to a temporary rise of serum specific radioactivity in healthy volunteers. 2. This short-term rise is followed by a more rapid decline and in some instances by oscillations of specific radioactivity. 3. Both these effects could be caused by secretion of thyrocalcitonin resulting from a rise of serum calcium concentration during calcium absorption from the gastrointestinal tract.