For the purpose of evaluating cause, frequency, type and seriousness of arrhythmias in dialysis patients, 14 chronic uremics, 8 on bicarbonate-dialysis, 6 on acetate-dialysis underwent a basal ECG, echocardiography and a Holter dynamic electrocardiography (ECGD) for a duration of 96 hours. Before and after dialysis PAO, body weight, serum electrolytes and arterial pH were controlled. In 11 patients (78%) supraventricular and ventricular arrhythmias were discovered of equal frequency and seriousness both in the inter and intra dialytic phase, even if more frequent in ventricular hypertrophic patients (IVSn) the complex ventricular arrhythmias (Lown greater than 2). The seriousness and frequency of ventricular and supraventricular arrhythmias in the dialytic phase did not seem to depend either on the type of tampon or on the presence or absence of cardiopathies while in the inter-dialytic phase the seriousness of ventricular arrhythmias seems to depend upon the presence of cardiopathies. The Authors conclude that the pathogenesis of arrhythmias in uremic patients on dialysis is multifactorial and that their elevated incidence makes the use of a Holter in these patients recommendable.