The variations of ionized and total calcium concentrations were studied during haemodialysis with high flux membrane using dialysate calcium concentrations of 1.63 and 1.76 mmol/l. The changes in ionized and total calcium were related to the predialytic calcium concentration difference between plasma and dialysate. The calcium changes were not influenced by the intradialytic pH variations. But the total calcium level increased more with the higher albumin concentration. Compared with previous studies, the calcium gradient concentration between plasma and dialysate was found to be the main factor responsible for the calcium variations observed during haemodialysis with high flux membrane.