When fluid intake is insufficient to match sweat losses, dehydration develops. It is well established that dehydration impacts unfavourably upon cardiovascular function, including cardiac output and peripheral blood flow (Gonzalez-Alonso et al., 1998). However, the limitations of cardiac electrophysiology in the dehydrated state are not known. In light of possible electrolyte imbalances, particularly when water deficit moves towards 7% of total body mass, it is worth considering the possibility of adverse conduction changes, as reflected within the electrocardiogram (ECG), may accompany electrolyte loss. In addition, the ECG can also be employed to investigate other cardiac limitations, such as myocardial ischaemia. Thus, in this project, set amongst a large dehydration study, basic cardiac conduction measures, and indicators of myocardial ischaemia (ST segment) were evaluated across a wide range of dehydration levels (1-7% water deficit).