In humans, sweating is one of the most variable routes of water loss. This thesis investigates sweat volumes and sweat composition during single training sessions. Chapter 4 investigates the variability of sweat losses and sweat composition between four training sessions. Forty-five able-bodied individuals (twenty males and twenty-five females) volunteered for the study and ranged in age from 18–65 years. Data was collected during the participants normal training sessions, consisting of various recreational pursuits. Sweat losses were determined by changes in body mass over the training session, and corrected for any fluid intake and urine losses during training. A sample of participants' sweat was obtained via adhesive gauze patches and analysed for sodium, potassium and chloride concentrations. Sweat samples were obtained from the shoulder, chest, forearm and thigh. The 95% confidence intervals determined from this study demonstrated the reliability of a single training session to estimate an individual's future hydration to within 2% of initial body mass.