Previous researchers have investigated the origin and subsequent spread of sinus rhythm through the atria, but unit recently these studies have been limited by the electrical mapping techniques available. A detailed appraisal of the literature is presented in Chapter 1. The thesis describes high-density, global right and left atrial activation patterns during sinus and paced rhythms in the porcine and human heart and when possible, correlates the findings with underlying atrial architecture. Central to the thesis methodology (Chapter 2) is the use of a novel electrophysiological mapping system. The EnSite 3000 system creates a three-dimensional reconstruction of cardiac chamber geometry. Onto this are superimposed high-density isopotential or isochronal maps. Sampling of the entire endocardium occurs 1200 times a second. A locator signal allows identification of anatomical structures and guiding of mapping catheters to specific sites of interest. In Chapter 3, non-contact mapping of sinus rhythm and sinus tachycardia in the porcine heart revealed an unpredictable shift in earliest endocardial activation during changes in heart rate. These findings are contrary to previous reports and have implications for techniques used to treat arrhythmias such as inappropriate sinus tachycardia. In Chapter 4, the spread of activation through the right atrial endocardium during sinus rhythm was correlated with endocardial dissection after the hearts were excised. A novel finding was the presence of conduction delay or block in the cavotricuspid isthmus, terminal crest and Triangle of Koch, which corresponded with changes in direction of myofibre orientation. The site of onset dictated the subsequent pattern of right atrial activation. In Chapter 5, left atrial mapping in patients with a history of atrial fibrillation was performed during sinus rhythm and coronary sinus pacing. Areas of conduction block and delay were seen in the inferior septum and posterior left atrium.