During plasma operation of ITER in the DT phase, tritium will be distributed in the different subsystems of the fuel cycle; tritium inventories within the systems are not constant, but vary as the gas moves through these systems during the burn and dwell periods. To evaluate the tritium content in each sub-system of the fuel cycle of ITER, a dynamic model for tritium inventory calculation was developed. The code reflects the design of each system in various degrees of detail; both the physical processes characteristics and in some cases the associated control systems are modeled. The amount of tritium needed for ITER operation has a direct impact on the tritium inventories within the fuel cycle subsystems. As ITER will function in pulses, the main characteristics that influence both the maximum value of tritium inventories in the systems and the rapid tritium recovery from the fuel cycle as necessary for refueling are discussed. Eventually the inventories in the Isotope Separation System (as the system with the highest tritium inventory) for short and long pulses and their dependence on the packing molar inventory are presented.