This thesis aims to identify physiological and biochemical variables, comparing sex, training status, age and maturity in sub-elite, endurance trained adolescents. Maximal lactate steady state was investigated and the effects of endurance training programmes measured.T he first study assessedth e reliability of absolute running speed, V02, and HR that correspond to the fixed blood lactate reference values of 2.0 and 2.5 mmo1. L"1 and the lactate threshold (LT) and found these measures to be reliable after endurance-trained adolescent runners completed two identical incremental treadmill tests within a 7-10 d period The second study was designed to determine the relationship between physiological variables and endurance running performance in this age group. Track-based, running performance times were available for 18 boys and 14 girls for the 800 m, and 16 boys and 13 girls for the 1500 m. The participants were tested using a step-wise incremental treadmill test and a Wingate anaerobic power test (WAnT) on separate occasions. The results from this study found that for the 1500m, running speeds corresponding to the fixed [BLa ] were a useful measure for assessing performance in endurance trained boys and girls. Unlike previous studies, peak V02 was not a significant physiological predictor of 1500m performance in either boys or girls. For the 1500 m performance in girls the anaerobic measure was no longer significant once variations in size or age had been taken into consideration. Whereas V VO2 peak and running economy may prove to be of some value when considering the 800m for boys, the running speed corresponding to a [BLa ] of 2.5 mmol-L-1 was the only meaningful physiological predictor variable for girls once differences in age and body size had been accounted for. The third study had three main objectives: (1) to identify the exercise intensity that corresponds to the (MLaSS) in adolescent, endurance trained runners, (2) to examine possible between sex differences, and (3) to compare the MLaSS with commonly cited fixed blood lactate reference variables. The participants were first tested using a step-wise incremental treadmill test to establish the blood lactate profile and peak VO 2. The running speed and % peak VO 2 at the MLaSS were not significantly different to those corresponding to the fixed [BL& ] of 2.0 and 2.5 mmol-L-1 (P>0.05). The % HR max at 2.5 mmol-L-1 was also not different to that at the MLaSS, whereas at 2.0 mmol-L-1 it was slightly lower (P<0.05). The running speed, % peak VO 2, and % HR max at the fixed [BLa] of 4.0 mmol-L-1 were significantly higher than those at the MLaSS (P<0.05). In conclusion, it is clear that the MLaSS corresponded to the relatively high exercise intensity in this sample of athletes. It would appear that the running speed, % peak VO 2, and % HR max at the MLaSS lies somewhere between the fixed [BLa ] of 2.0 and 2.5 mmol"L-1. These results confirm earlier work that has suggested a fixed [BLa ] of 2.5 mmol-L-1 may be used with young people' to assess and monitor endurance running performance in place of the more commonly used 4.0 mmol-L-1 that has received so much attention in adult-based studies. The fourth study examined the effect of exercise training on endurance performance, blood lactate profile in relation to running speed (RV) and cardio respiratory function (peak V02) in adolescent runners. This study demonstrated that resting HR, LT and 1 Use of the expression young people is increasingly common since the publication of the text, Young People and Physical Activity by Armstrong and Weisman in 1997. It is used within this document to generically represent the 6 to 18 year age group. 11 RV, HR, V02 and peak V02 at LT were significantly influenced by endurance training. When running time, running velocity and run performance time pre and postintervention were included in the analysis, the intervention did not have a significant effect on peak VO2. When percentage body fat was included as a covariate, there was a positive association with pre and post-training for all groups. The conclusion from these data is that maturity and training both have an effect, especially at supra suggested training levels. The results of the four inter-linked studies support an age-related increase in endurance in aerobic and anaerobic performance and indicated significant differences between boys and girls. From a coaching viewpoint the results reveal that, from the age of 14 to 18 years, runners should be introduced to high intensity training and that changes to the format of middle distance running performance in adolescent competition are recommended.