Introduction: Arterial ammonia concentrations increase acutely during the anhepatic phase of a liver transplantation (LTx) and return to baseline within 1 h after reperfusion of a functioning liver graft. So far, this return to baseline has solely been attributed to hepatic ammonia clearance. No data exist on the potential contribution of altered renal ammonia handling to peritransplantation ammonia homoeostasis. Aim: The present study investigated the consequences of a hepatectomy and subsequent implantation of a partial liver graft on arterial ammonia concentrations and urinary ammonia excretion during a living donor liver transplantation (LDLTx). Methods: Patients with end-stage liver disease undergoing LDLTx were selected. Samples of arterial blood and urine were taken before, during and 2 h after the anhepatic phase. Differences were tested using Wilcoxon's test. Results are given as median and range. Results: Eleven adult patients undergoing an LDLTx were included. Before hepatectomy, arterial ammonia concentrations were 89 muM (40-156 muM), increasing to 146 muM (102-229 muM) (P<0.001) during the anhepatic phase and returning to 79 muM (46-111 muM) (P<0.01) after reperfusion. Urinary ammonia excretion was initially 1.06 mmol/h (0.02-6.00 mmol/h), increasing to 3.81 mmol/h (0.32-12.55 mmol/h) (P=0.004) during the anhepatic phase and further increasing to 4.00 mmol/h (0.79-9.51 mmol/h) (P=0.013) after reperfusion. Conclusion: The kidney significantly increased urinary ammonia excretion during the anhepatic phase, which was sustained after reperfusion, contributing to the rapid decrease of ammonia concentrations. Accordingly, the plasma ammonia concentrations measured directly after LTx cannot simply be used as a read-out of initial liver graft function.