The purpose of this thesis was to examine the effect of fluid and/or energy deficits on: 1. the validity of plasma, urine and saliva hydration indices to identify acute and chronic changes in hydration status; 2. salivary immunoglobulin A (s-lgA) at rest and after exercise; and 3. endurance performance. Modest hypohydration (2-3% body mass loss) evoked by acute (1.5 h) and chronjc exercise and fluid restriction ( 48 h) was identified by plasma, urine and saliva osmolality and sa liva flow rate. Using upper euhydrated values, recommended by the American College Sports Medicine for plasma and urine osmolality (>290 and 700 m0smol·kt1, respectively) and a proposed saliva osmolality threshold (>61 mOsmol·kg-1) , greater than 75% of individuals were correctly identified as hypohydrated following exercise and fluid restriction protocols. Modest hypohydration resulting from fluid and energy restriction was also identified by saliva and urine indices but not by plasma osmolality. Hydration indices were unable to indicate hypohydration associated with energy restriction alone. This highlights the importance of adequate dietary intake when monitoring hydration changes using these markers, particularly plasma osmolality. s-IgA secretion rate was decreased after 48 hours of combined fluid and energy restriction. This was most probably due to decreased saliva flow rate and impaired synthesis and/or secretion of s-IgA which may increase individuals' susceptibility to infection. Compared with pre time trial (TT) s-IgA was not lower following a 30 minute treadmill TT highlighting a limited effect of this exercise bout and prior fluid and/or energy restriction on s-IgA responses to exercise. Encouragingly, s-IgA was shown to be normalised after six hours of rehydration and refeeding. TT performance was s ignificantly decreased following 48 hours of energy restriction and combined fluid and energy restriction but not after fluid re striction alone; supporting a limited effect of modest hypohydration on endurance performance in a temperate environment.