Running economy (RE) is defined as the rate of oxygen consumption at a given submaximal running velocity. Whilst the concept of running economy is well documented, little information is available about the to daily variation in RE, variation in RE within prolonged running bouts and the effects on RE of exercise-induced dehydration. Thus, the principal aim of this research was to investigate these aspects and attempt to contribute further knowledge and understanding of RE. The purpose of the first study (Chapter 4), was to investigate the daily variation in RE in twenty one habitually active Singaporean men (V0₂max = 51.6 ± 5.8 ml.kg⁻¹ min⁻¹). The RE was measured over three consecutive days during treadmill running at 3.33 m.s⁻¹, and at running speeds representing relative exercise intensities of 60% and 85% V0₂max (randomly assigned in a counterbalanced design). The running bouts were of 10 min duration at each speed, with 5 min rest between each running bout. The group mean (±SD) VO₂ during submaximal running at 3.33 m.s⁻¹ (82 ± 5.7% V0₂max) was 44.5 (±2.1), 43.8 (±2) and 44.2 (±2) ml.kg⁻¹ min⁻¹ for days one, two and three respectively. The mean coefficient of variation (CV) for RE at 3.33 m.s⁻¹ was 2.54 % (range = 0.24 - 5.41%). The group mean VO₂ during submaximal running at 60% and 85% VO₂max was similarly invariant over the three consecutive days. The results showed no differences in daily RE for the group (p > 0.05). The results suggest that for the individuals in this study RE does not change on a daily basis.