This study was designed to assess the influence of magnesium on circulating plasma immunoreactive parathyroid hormone (iPTH) in end-stage renal disease. 20 patients receiving regular hemodialysis therapy underwent plasma measurements of iPTH and 25-hydroxycholecalciferol (25-OHCC) concentrations before and 10 weeks after the magnesium concentration in the dialysate was increased from 0.75 to 1.50 mmol/l. This resulted in a 36% rise in the mean predialysis plasma magnesium concentration from 1.25 to 1.70 mmol/l (p less than 0.001) and a 23% fall in the mean plasma iPTH concentration from 546 to 418 ng/l (p less than 0.001). Mean plasma concentrations for calcium, phosphate, and 25-OHCC also decreased, but these changes were not significant. In conclusion, we have demonstrated that a rise in plasma magnesium concentration from elevated to significantly higher levels reduces circulating plasma iPTH in normocalcemic uremic patients with initially both normal and raised plasma PTH levels.