Myoelectrical activity was recorded from the human antrum on 136 occasions using a monopolar mucosal electrode in preoperative and post-vagotomy patients, and bipolar serosal electrodes in post-cholecystectomy patients, and the incidence of antral arrhythmias observed. Arrhythmias of long duration were observed in five out of 62 patients after vagotomy and one out of 10 patients after cholecystectomy, but not in preoperative patients. They were characterised by slow waves with increased frequency, variable period, amplitude, and wave shape and were associated with periods of slow wave inhibition. None of the patients with an arrhythmia had any symptoms of disordered gastric motility. Antral arrhythmias could also be induced in some patients by the administration of insulin, secretin, cholecystokinin-pancreozymin, glucagon, and pentagastrin. The appearance of antral arrhythmias is probably due to a relative increase of sympathetic over parasympathetic activity. The maximal slow wave frequency observed was 8.3 cycles per minute and it is probable that in vivo human antral smooth muscle has a maximum frequency above which it cannot be driven.