Calcium homeostasis was longitudinally followed in serum and urine throughout normal pregnancy and the puerperium in 23 healthy women. From the 14th week of gestation, samples were obtained every fourth week until the 38th week. Post partum samples were obtained on the fifth day and after eight weeks. In the serum the total calcium decreased continuously during pregnancy. The ionized calcium and phosphate levels remained unchanged and within the reference interval for non-pregnant women. The alkaline phosphatase level progressively increased and high levels were found at term. The magnesium and hematocrit values remained below, whereas the calcitonin level remained just above the reference interval throughout pregnancy. The parathyroid hormone was low initially and increased towards term but within the reference interval. The urine excretion of calcium was constantly high, close to the upper reference limit, and renal function was slightly improved. At the last sampling eight weeks after delivery, all values were within normal limits for non-pregnant women. Calcium homeostasis is considerably changed during pregnancy and non-pregnant reference limits are not often valid.