During the startup phase of the thorium high-temperature reactor (THTR)-300, control rod reactivity measurements are performed to meet the demands of the licensing authorities as well as to adjust the operating instrumentation. From several possibilities the inverse kinetic method has been chosen because of its easy implementation, high stability against disturbing signals, and great accuracy. An outline of the basic assumptions of various methods together with tests on the Kritische Anlage Hochtemperaturreaktor are discussed, the technical equipment for the THTR is described, and results of measurements for various rod configurations are given. Finally, the transition from the special startup equipment to the standard operating instrumentation is demonstrated.