During the early post-transplantation period, the limitations of monitoring current tacrolimus dose with classic pharmacokinetics (PK) have been demonstrated in liver-transplant recipients. Evaluation of the pharmacodynamics (PD) using calcineurin activity (CNA) has been proposed to optimize tacrolimus dosing. The aim of the present study was to determine the time of maximal inhibition of CNA, to explore the relation between exposure to tacrolimus and CNA, and to analyze its variability. Blood was drawn from 14 patients 0, 2, 3, 4, 6, and 9 hours after tacrolimus intake on post-transplantation days 8, 21, and 90 to measure blood tacrolimus concentrations using the EMIT 2000 assay and CNA in peripheral blood mononuclear cells. Tacrolimus and CNA data were obtained for 33 blood-sample collection sessions and analyzed using a population approach. Three models were built to describe tacrolimus PK, CNA kinetics, and the relationships between the area under the CNA-time curve (AUC12effCNA) and AUC12Tacrolimus or CminTacrolimus. Coagulation factor V and whole/split liver graft were identified as covariates influencing tacrolimus clearance. Indeed, apparent tacrolimus clearance rose by 14% when factor V increased by 10% and was threefold higher in patients with whole-liver grafts. The median maximal inhibition of CNA was reached 4 hours after tacrolimus intake on days 8, 21, and 90 and represented an 18% drop in CNA compared with activity at drug intake. The variability of the PK-PD relationship was minimal when using AUC12Tacrolimus. The large variability of the PD parameters (coefficient of variation was 89%) that linked AUC12effCNA to AUC12Tacrolimus indicates that monitoring tacrolimus concentrations may not be adequate to control CNA. Measuring CNA in peripheral blood mononuclear cells 4 hours after tacrolimus intake during the first 3 months after liver transplantation could be a means to improve tacrolimus monitoring and thereby avoid acute graft-rejection episodes.