Attempts have been made to account for the increased propensity of upper respiratory tract infections (URTI) in regularly training individuals, by focusing on the effect of exercise on the primary mucosal immunoglobulin secretory immunoglobulin A (sIgA). However, the pathophysiological significance of observed changes in sIgA levels remains unclear, as indeed does the existence of increased URTI in athletes. The aim of this work was to examine the use of sIgA as a marker of exercise-induced immunomodulation and its possible role in protection against URTI. Before these questions could be addressed the optimum method for saliva collection, and the ideal terms for expression of sIgA levels were investigated. Experimental data advocate the use of the passive dribbling method above the use of salivettes. Salivettes were found to result in an underestimation of both saliva flow rate and protein concentrations as a consequence of limited absorption (3 ± 1ml) and variable retention of the sample (49.1 ± 24.9%). Secretory IgA levels are commonly expressed in terms of secretion rate which is the product of saliva flow rate and sIgA concentration. Examination of the relative contribution of these two factors to secretion rate revealed that exercise-induced changes in saliva flow rate (-51 %) matched the changes in secretion rate (-51 %), whereas changes in sIgA concentrations (+4%) did not. It was concluded that changes in saliva flow rates have an important role with the occurrence of symptoms associated with URTI whether induced by infective or inflammatory factors. Epidemiological data from others on the incidence of symptoms associated with URTI in marathon runners have provided evidence on the incidence of URTI in athletes. However here, reported symptoms associated with URTI were most common during the race suggesting that an infective agent was not involved. Examination of the effect of marathon running revealed a non-significant decrease in saliva flow rate (-27.7 ± 15.8%). A final study investigated the effect of increased ambient temperature, and the possibility of fluid replacement as a intervention strategy against exercise-induced decreases in saliva flow rate. This study revealed that exercise reduced saliva flow rate exercise in the heat exacerbated this and that fluid replacement tempered the exercise-induced decrease. Changes in saliva flow rate were found to be associated with changes in plasma volume. The overall conclusions of this thesis are that innate defence mechanisms such as saliva have a role to play in conferring defence against potential pathogens, and therefore warrant further investigation. It appears from data presented in this thesis that saliva flow rate is affected by exercise, perhaps to a greater extent than sIgA concentration. Changes in saliva flow rate with exercise may have a role to play in the purported increased incidence of URTI reported by athletes, and fluid replacement may provide an effective strategy against this exercise-induced decrease.