SUMMARYIsorhythmic atrioventricular dissociation occurs frequently under general anaesthesia but is rarely recognized without continuous electrocardiographic monitoring. Since this arrhythmia was observed in about 50 per cent of the cases during the onset of the neuromuscular blocking action of alcuronium or pancuronium during halothane anaesthesia, the opportunity was taken to investigate the haemodynamic parameters during atrioventricular dissociation. In nine patients the haemodynamic parameters during atrioventricular dissociation as compared to sinus rhythm were characterized by an absence of significant differences in cardiac rate, by a mean lowering in arterial mean pressure of 14 per cent and by a mean decrease in cardiac output of 15 per cent. The calculated peripheral resistance remained unaltered.