The effect of experimental hypoxia on the automatic activity of the sinus node and sinoauricular transmission of excitation was studied in 17 isolated preparations of rabbit hearts. A greater frequency of the shifts of sinoauricular delay was noted under hypoxia. Local electrophoretic applications of adrenalin demonstrated that the shifts of the delay occur because of the shifts of pacemakers. The mean frequency of the automatic activity recorded in the atria falls to zero between 6 and 20 min of hypoxia. Hypoxia causes the development of sinoauricular blocks and atrial arrhythmias in the form of individual extrasystoles, high-frequency discharges, or alternations of series of impulses with "mute" periods. The increased tendency towards arrhythmias in the sinoauricular zone is, according to the authors, due to the growing heterogeneity of the duration of action potentials that increases more than three-fold by the 10th minute of hypoxia. The possible reflection of the excitation wave ("echo") from the sinus node zone with an inhibited excitability was demonstrated.