The purpose of this study was to compare fluid balance between a resistance and aerobic training session, in elite rugby players. It is hypothesised that resistance exercise will result in a higher prevalence of over-drinking whereas during the aerobic session under-drinking will be more prevalent.As with previous fluid balance studies, this was an observational study. Twenty-six players completed the resistance training session and twenty players completed the aerobic training session. All players were members of an elite rugby union squad competing in the southern hemisphere’s premier competition. For both sessions players provided a pre-exercise urine sample to determine hydration status, pre- and post-exercise measures of body mass and blood sodium concentration were taken and the weight of drinks bottles were recorded to calculate sweat rates and fluid intake rates. Sweat patches were positioned on the shoulder of the players and these remained in place throughout each training session,and were later analysed for sodium concentration.The percentage of sweat loss replaced was higher in the resistance (196 ± 130%) than the aerobic training session (56 ± 17%; P=0.002).Despite this, no cases of hyponatremia were detected. The results also indicated that over 80% of players started training in a hypohydrated state.Fluid intake appears to differ depending on the nature of the exercise session. In this group of athletes, players did not match their fluid intakes with their sweat loss, resulting in over-drinking during resistance training and under-drinking in aerobic training. Therefore, hydration strategies and education need to be tailored to the exercise session. Furthermore, given the large number of players arriving at training hypohydrated, improved hydration strategies away from thetraining venue are required.