A circadian variation in serum calcium, albumin and PTH concentration in normal subjects has been demonstrated. The levels of the three blood constituents were remarkably constant during the day, but striking night and early morning changes occurred. Serum calcium levels were highest at 8:00 p.m. and reached a nadir between 2:00 and 4:00 a.m. Serum albumin levels were parallel to those of serum calcium. PTH levels began to rise after 8:00 p.m., reached the highest levels between 2:00 and 4:00 a.m., and fell to baseline values by 8:00 a.m. The nocturnal fall in serum calcium levels appears to be secondary to dilution of serum proteins by increasing blood volume. The nocturnal rise in PTH levels appears to be independent of serum calcium levels within the normal range but it can be abolished by induced hypercalcemia. Serum phosphate levels were lowest between 8:00 a.m. and 10:00 a.m. and highest between 2:00 a.m. and 4:00 a.m. The data presented suggest that circadian changes in serum phosphate levels are not mediated in toto by parathyroid hormone but they are exaggerated when the secretion of this hormone is inhibited. They are independent of growth hormone levels and activity but they are greatly modified during a prolonged fast.