The total content of calcium and protein was studied in 50 patients during the initial 5 days after surgery for their congenital or acquired heart diseases conducted under extracorporeal circulation; in half of them the content of the dialysing fraction of total calcium and plasma albumins were also examined. The study has demonstrated that during the 1st postoperative day the content of the total and dialysing calcium is elevated which is due to the administration of calcium for the neutralization of citrate during blood transfusions. The greatest fluctuations of these values (both decreases and increases) were observed when the heart-lung machine was primed with blood containing 18.2--23mEg Ca++ per 1 1 of citrate blood. Within the early 5 postoperative days no restoration of the impaired calcium balance is observed in these patients, while the blood content of calcium is returning to the preoperative level. The blood content of calcium was studied after repeated injections of various doses of calcium. The most rational method proved to consist in repeated injections of 5ml of 10% CaCl2 solution every 30 min--1 hour which resulted in a moderate elevation of the blood content of the total and dialysing calcium.