Nonlinear mixed effects modeling (NONMEM software version VI and Intel Fortran (version 8) compilation tool) will be used to analyze the dose-concentration-time data for Tac using a population approach. The pharmacokinetic profile data will be used to develop a pharmacokinetics model including the effect of major covariates (e.g. age, gender, body size, renal function, hematocrit, acute rejection status etc.) on CL/F and V/F from the 24-hour pharmacokinetics investigations. If applicable additional trough concentration data from the routine follow-up of the patients in the study will be used to develop the model.