The experiments were carried out on preparations from rabbit right atrium, containing the sinoatrial node, the interatrial barrier, the atrioventricular node and the His-bundle. Surface electrodes were used to record the atrial and His-electrograms. The conduction time (AH-interval) was measured and the functional refractory period of the node was determined during normoxia (95% O2, 5% CO2) and hypoxia (30% O2, 65% N2 and 5% CO2) for 45 min. Hypoxia causes a considerable delay in conduction. This delay is greater for the shorter coupling-intervals and at the higher frequencies of the imposed rhythm. The shifting of the conduction times towards the higher values during hypoxia is not parallel compared with hypoxia. In the case of hypoxia the refractory curve of the A-V node is situated higher than the curve during normoxia, owing to the greater duration of the H1H2 intervals. The functional refractory period is increased and shifted toward the longer coupling-intervals. The results obtained show that during hypoxia the functional refractory properties of the atrioventricular node are impaired, which, combined with the delayed conduction through it, creates conditions for serious disturbances in the function of the atrioventricular conduction system.