Inadequate atrial hypothermia and subsequentischemic injury have been recognized as the major causes of supraventricular arrhythmias (SVAs) and conduction defects following cold chemical cardioplegia. This study was designed to assess the effects of right atrial cooling (15°-20°C) during cardioplegic arrest upon the incidence of postoperative SVAs and conduction defects in 40 consecutive patients undergoing isolated aortic valve replacement. Atrial preservation was ensured by combining systemic (24°C) and topical hypothermia with snared double caval cannulation during arrest. Myocardial temperatures in the right atrial septum and anterior wall of the right ventricle were recorded before and after each cardioplegic infusion and upon release of caval tapes. Postoperatively, the incidence of SVAs and conduction defects was assessed by continuous rhythm monitoring, bipolar atrial electrograms and, in ten patients, 24-h Holter recordings during the first postoperative day. With the venae cavae snared, temperatures in the right atrial septum were not significantly different from those measured simultaneously in the right ventricle. Release of caval tapes resulted in right atrial temperatures increasing to systemic temperature (from 17.1 ± 2.9°C to 25.9 ± 5.6°C [m ± SD]; P < 0.01). Atrial rewarming between cardioplegic infusions did not exceed 2.9° ± 3.2°C. Postoperatively, four patients (10%) developed sustained atrial fibrillation. One additional patient had a single episode of paroxysmal atrial fibrillation and two patients experienced asymptomatic episodes of junctional rhythm. We conclude that the combination of simple measures, among which double caval cannulation with snaring is critical, results in effective cooling of the right atrial septum during cardioplegic arrest and is associated with a relatively low incidence of rhythm abnormalities postoperatively. © 1987 Springer-Verlag.