The pathogenesis of cardiac arrhythmias in the pre-hospital phase of acute myocardial infarction is considered in the framework of four arbitrary divisions of the conduction system: the sinus node, the internodal and interatrial pathways, the atrioventricular (A-V) nodes and the His bundle with its proximal branches. Under each heading the elements of pathogenesis are discussed relative to local anatomy, vascular supply, neural control and blood-borne factors. These divisions are made purely for the sake of discussion; an accurate assessment of their varying presence and interplay in the individual patient is the challenging responsibility of the physician who deals with the problem of arrhythmias in acute myocardial infarction. © 1969.