The studies described in this thesis were undertaken to investigate the physiological responses to short-term training stress in male distance runners. The aim of the thesis was to identify practical physiological markers of training stress that could identify acute fatigue and hence determine recovery status within a physical training programme. Study 1 investigated the physiological responses of 6 well-trained runners and 6 active but untrained individuals to 15 min of moderate intensity (70 % VO,max) treadmill running performed immediately before, 1,48 and 96 h following a treadmill run to exhaustion at 75% VO,max (Exh75%). The trained group ran for 107.1 ± 6.5 min before exhaustion in comparison to untrained group who ran for 60.6 ± 3.9 min (p