Regulation of magnesium balance is achieved by a steady-state mechanism in which intake and output are maintained at an equal level. Dietary magnesium intake, total and ionized plasma magnesium levels, and urinary magnesium were assessed in 46 renal transplant recipients treated with cyclosporine, nine transplant recipients who had never been on cyclosporine, and 31 healthy volunteers. Dietary magnesium intake [13.5 (11.0-15.1) mmol/day vs 13.0 (11.1-16.0) mmol/day and 13.7 (11.4-16.7) mmol/day, respectively; median and interquartile range] and urinary magnesium excretion [4.31 (3.57-5.89) vs 4.39 (3.56-6.02) and 5.01 (3.73-6.01) mmol/day, respectively] were similar in renal transplant recipients treated with cyclosporine, transplant recipients who had never been on cyclosporine, and control subjects. Total [0.74 (0.70-0.78) vs 0.80 (0.74-0.84) and 0.81 (0.79-0.87) mmol/l), respectively] and ionized [0.49 (0.46-0.52) vs 0.53 (0.50-0.58) and 0.54 (0.52-0.59) mmol/l, respectively] plasma magnesium were significantly lower in renal transplant recipients on cyclosporine than in transplant recipients without cyclosporine, and healthy controls. These observations indicate a modified magnesium steady state in renal transplant recipients treated with cyclosporine.