P lasma concentration of immunoreactive parathyroid hormone (IPTH) was measured in 18 patients who had been on a hemodialysis program for longer than 6 months. A negative correlation was found between the predialysis plasma concentration of IPTH and the mean concentration of calcium in the dialysate previously used: plasma concentrations of IPTH were higher in patients dialyzed against a calcium concentration between 4.9 and 5.6 mg/100 ml than in patients dialyzed against a calcium concentration of 6.0 mg/100 ml or more. Plasma concentrations of IPTH also were higher in patients with bone disease than in patients without bone disease. Furthermore, a positive correlation was found between predialysis plasma concentrations of IPTH and calcium, and between mean predialysis concentration of IPTH and phosphate. To obviate the possibility that individual differences in susceptibility could have accounted for the observed effects of plasma phosphate and of dialysate calcium, a 2 × 2 factorial study was conducted in seven of these patients to examine the independent effects of perturbation of each of these factors. It was observed that plasma concentration of IPTH was lowest with the combination of high dialysate calcium and low plasma phosphate, highest with the combination of low dialysate calcium and high plasma phosphate, and intermediate with the two other combinations. It is concluded that both dialysate calcium and plasma phosphate are important determinants of parathyroid function in these patients.