This study evaluated the training adaptations in female adolescent cross-country runners over two training seasons. Although the effects of endurance training in this population are understood, the specific adaptations resulting from pre-season and in-season training are not clear. The physiological changes occurring over these two training seasons in 10 female runners (15.7 ± 0.8 yrs, 165.6 ± 5.6 cm, 53.8 ± 6.1 kg) were assessed. Maximal exercise responses were measured during a continuous graded exercise test and submaximal exercise responses were quantified at 7 and 8 mph. Isokinetic muscle strength was assessed at four movement speeds (60-240°/sec) and muscle power was calculated using vertical jump height. Body composition also was assessed. HR at maximal and submaximal exercise declined during pre-season training and then increased during in-season training. Blood lactate [BLa] measurements at 7 and 8 mph decreased following summer training. Whole body bone mineral content (BMC) and bone mineral density, and femoral head BMC increased. In conclusion, it appears that high volume, low intensity training lowers [BLa] with no corresponding change in oxygen utilization in highly trained female adolescents. Running also appears to have a positive impact on bone mass in female adolescents. / School of Physical Education, Sport, and Exercise Science