The purpose of this thesis was to (1) examine the psychological morbidity associated with atrial fibrillation (AF), and (2) determine the effects of acute mental and postural stress, and hydration status on indices haemorhelogy, endothelial function, and platelet reactivity. Symptoms of depression (BDI scores > 10) persisted in 38% of patients with AF, with elevated state and trait anxiety (STAI score >40) being reported in 28% and 38% of patients, respectively. No significant differences in depression, state anxiety, and QoL were observed between AF and hypertensive patients; however, AF patients displayed higher levels of trait anxiety. Multiple regression analysis demonstrated baseline depression scores provided the best independent prediction of future QoL. Acute mental and postural stress yielded significant changes in haemodynamics and haemorhelogy, in addition to increasing biomarkers of endothelial dysfunction and platelet reactivity. Increased hydration status reduced blood pressure and markers of endothelial perturbation and platelet morphology both at rest and in response to the stress tasks. In conclusion, AF patients display comparable levels of depression and anxiety to patients following a myocardial infarction. In addition, behavioural activities such as mental and postural stress may implicated in the pathogenesis of acute coronary syndromes through the development of a prothrombotic state.