The electrophysiological and ultrastructural alterations that occur in chronic diabetes mellitus were studied in isolated, superfused atria including sinoatrial (SA) and atrioventricular (AV) nodes that were obtained from rabbits subjected to alloxan-diabetes for 100 days. Transmembrane action potentials were recorded simultaneously with the His bundle electrogram. The results were compared with corresponding values observed in atria from normal, non-diabetic rabbits. The abnormalities of automaticity and conduction found in atria of alloxan-diabetic rabbits included lower sinus rate, longer poststimulatory sinus-node recovery time, inhomogeneity of atrial conduction and the occurrence of AV nodal block. Ultrastructural analysis of the diabetic preparations revealed intracellular accumulation of abundant glycogen and various mitochondrial abnormalities but a normal appearance of the small coronary vessels. It was concluded that chronic alloxandiabetes mellitus may directly alter the electrophysiological and ultrastructural properties of nodal and atrial fibers in the absence of vascular damage.