In order to study the influence of calcitonin as a calcium lowering factor during thyroid surgery, we measured calcitonin, calcium, parathyroid hormone, phosphate and alkaline phosphatases in serum of 10 euthyroid patients undergoing minor thyroid surgery. A significant increase was found in serum calcitonin during the first 48 hours postoperatively (p less than 0.01), as well as a transient decrease in serum calcium ( less than 0.01). Serum parathyroid hormone remained almost constant. The results suggest that the release of calcitonin from the traumatized thyroid tissue is the first step in the development of hypocalcaemia in patients undergoing thyroid surgery rather than an impairment of the parathyroid function.