Using a new ion-selective electrode, plasma concentration of ionized magnesium was measured in nine adult patients undergoing orthotopic liver transplantation. Baseline plasma ionized magnesium (IMg2+) concentration (0.49 +/- 0.07 mmol/L) was slightly below normal values (0.55-0.66 mmol/L, 95% CI): Six patients had ionized hypomagnesemia and two of these had total hypomagnesemia. Ionized IMg2+ concentration progressively decreased during the dissection (0.45 +/- 0.07 mmol/L, p < 0.05) and anhepatic stage (0.38 +/- 0.07 mmol/L, p < 0.05) and returned toward baseline values by 2 hours after graft reperfusion. Plasma ionized calcium levels and acid-base status were maintained within normal limits during surgery. Serum citrate concentration increased during the dissection (0.58 +/- 0.60 mmol/L) and anhepatic stages (1.18 +/- 0.78 mmol/L), the result of transfusion of citrate-rich blood products in the absence of adequate hepatic function, and gradually returned toward baseline values after graft reperfusion. IMg2+ concentration inversely correlated with the plasma citrate concentration (r2 = 0.54). The results of this study demonstrate that ionized hypomagnesemia invariably occurs during liver transplantation and suggest that this derangement may be a clinical concern, because magnesium is an important cofactor for the maintenance of cardiovascular homeostasis. The data further suggest the clinical importance of supplementation with magnesium based on the monitoring of plasma IMg2+ concentration.