1. The diurnal rhythms of urinary calcium excretion were studied in healthy subjects under four different regimens in which diet, activity and posture were varied. The excretory rhythms observed were compared with those of potassium, obtained under the same conditions. 2. When the subjects were active throughout the day and night and were either fasting or eating standard three-hourly meals, an intrinsic calcium rhythm was isolated with maximum amounts excreted between 06.00 and 09.00 hours. 3. Continuous supine bedrest was found to have a specific effect on calcium excretion. The total 24 h excretion was markedly increased compared with that found on constant activity but on the same dietary regimen, and 78% of the increase occurred between 09.00 and 18.00 hours.