Th e aim of this study was to determine the performance indices (ideal sprint time -IS, total sprint time -TS, and performance decrement -PD) of two repeated sprint test (RST) and to examine their relationships with the aerobic fi tness of young tennis players. Fift een young (age 14.7±1.0 yrs) tennis players performed three tests: an aerobic power test (20 m shutt le run), and two diff erent RST protocols (12×20 m and 12×10 m runs). Peak heart rate was signifi cantly higher in the 20 m protocol compared to the 10 m protocol while no signifi cant diff erence was found in the PD of the two RST protocols. Signifi cant positive correlations were found between the ISs and the TSs (r=0.946 and r=0.932, respectively), but not between the PDs of the two RST protocols. Signifi cant negative correlations were found between TS and IS and aerobic fi tness during the 10 m protocol (r=-0.594 and r=-0.595, respectively) and the 20 m protocol (r=-0.757 and r=-0.716, respectively), but not between PD and the aerobic fi tness in both RST protocols. Both short and long RST protocols represent similar anaerobic capabilities. In addition, the aerobic energy system serves as a signifi cant factor in both RST protocols. However, it seems that the involvement of the aerobic system is more signifi cant in the long than in the short repetition RST protocol.