Calcium transfer during intermittent peritoneal dialysis (IPD) was studied in four groups of patients with chronic renal failure. Group I dialyzed with a dialysis fluid concentration of 2.16 mmol/l. Group II dialyzed with the same calcium content, but with a hyperosmotic solution. Group III dialyzed with a high calcium concentration, 4.47 mmol/l in dialysis fluid and in group IV the patients dialyzed 5 cycles against a calcium-free solution followed by 5 cycles with normal calcium content. In the first three groups a significant rise in serum total calcium and ionized serum calcium took place. At balance studies a positive net transfer of calcium from dialysis fluid to blood was demonstrated, the largest transfer occurring in group III. High osmolality in dialysis fluid is of no importance for the calcium transfer. In group IV a significant fall in serum total calcium during the first 5 cycles was removed. The loss of albumin to the dialysate did not affect the calcium transfer as well as the changes in serum albumin and pH did not influence the calcium state during IPD.