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This study investigated training loads, injury rates, and physical performance changes associated with a field conditioning program in junior and senior rugby league players. Thirty-six junior (16.9 [95% confidence interval: 16.7–17.1] years) and 41 senior (25.5 [23.6–27.3] years) rugby league players participated in a 14-week preseason training program that included 2 field training sessions each week. Subjects performed measurements of standard anthropometry (height, body mass, and sum of 7 skinfolds), muscular power (vertical jump), speed (10-, 20-, and 40-m sprint), agility (L run), and maximal aerobic power (multistage fitness test) before and after training. Improvements in agility, muscular power, and maximal aerobic power were observed in both the junior and senior players following training; however, the improvement in maximal aerobic power and muscular power were greatest in the junior players. Training loads and injury rates were higher in the senior players. These findings demonstrate that junior and senior rugby league players adapt differently to a given training stimulus and that training programs should be modified to accommodate differences in training age.