The role of hipoxia on A-V conduction was studied in the patients with severe respiratory insufficiency. Intervals of A-V conduction were measured under basal conditions, breathing room air and with increasing atrial rates. Results are compared with those obtained after the administration of 99% oxygen. A-V conduction times were normal in all the patients while breathing room air and in response to atrial pacing. With oxygen administration most patients had a decrease in A-V conduction at the expense of a prolonged A-H time, without significant alteration of the other time intervals. A-V conduction in chronic hipoxia with a PaO2 above 28 mm Hg is not delayed. Intranodal conduction delay with oxygen administration may be due to a decrease in sympathetic nerve stimulation.