Serum total magnesium (TMg) measurements in dialysis patients are variable, with some groups reporting hypermagnesemia and some hypomagnesemia. It had not been possible to measure the biologically active fraction, ionized magnesium (IMg2+). The authors utilized an ion-selective electrode to measure IMg in 26 hemodialysis patients and 10 peritoneal dialysis (CAPD) patients and compared the results with those from 66 age matched control subjects. Dialysate magnesium was 0.375 mM/L for the hemodialysis and 0.25 mM/L for the CAPD patients. When compared with control subjects, both hemodialysis and CAPD patients had significantly lower IMg2+ (0.55 +/- 0.02 and 0.50 +/- 0.02 vs. 0.60 +/- 0.004 mM/L; p < 0.05) and greater or normal TMg values (0.99 +/- 0.04, different at the p < 0.001 level, and 0.85 +/- 0.04 vs. 0.84 +/- 0.008). Ionized calcium (ICa2+) values were similar for all three groups (1.15 +/- 0.02 and 1.21 +/- 0.04 vs. 1.17 +/- 0.01), resulting in increased mean ICa2+/IMg2+ ratios (2.14 +/- 0.07 and 2.42 +/- 0.06 vs. 1.95 +/- 0.02 for the control subjects; p < 0.05). The percent of total magnesium that was ionized (%IMg2+) was low in both the hemodialysis and CAPD patients (55.6 +/- 0.93 and 59.2 +/- 1.05) compared with that of control subjects (72 +/- 0.61; p < 0.05). IMg2+ values correlated with TMg values in both hemodialysis (r = 0.93; p < 0.0001) and CAPD (r = 0.92; p < 0.0001) patients did not correlate with age, time on dialysis, weight, fasting cholesterol or triglyceride, albumin, blood urea nitrogen (BUN), creatinine, hematocrit, phosphate, or PTH values.(ABSTRACT TRUNCATED AT 250 WORDS)