The effect of calcitonin (CT) on small intestinal calcium absorption was studied using Thiry—Vella loops in one intact sheep, one intact pig and three parathyroidectomized pigs. Net calcium absorption rate was measured after recirculating through the loop a known volume of a solution containing calcium and polyethylene glycol 4000. Calcitonin was infused intravenously and its effect on the net calcium absorption rate was measured. When relatively high doses of CT (10 mu./min/kg) were infused for up to 45 h, there was an initial rise in net calcium absorption associated with hypocalcaemia, followed by a marked reduction in calcium absorption. When small doses of CT (0·5 mu./min/kg) were infused for 100 h, the increase in the net absorption rate was not observed or was less marked, but there was a significant reduction in net calcium absorption 2 days after the CT infusion was stopped. A reduction in net calcium absorption rate was seen both in intact and parathyroidectomized animals. In one experiment in which the true absorption rate of calcium from lumen to blood was measured using 47Ca, a reduction in unidirectional transfer of calcium from lumen to blood was seen 2 days after the CT infusion was stopped. The possible mechanism of this action of CT and its significance in calcium homeostasis during the ingestion of a high calcium diet is discussed.