Changes in arterial Pco2(Paco2) and body temperature normally occurring at the moment of birth may play a role in the initiation and maintenance of continuous breathing. To clarify these mechanisms, five chronically instrumented fetal lambs were connected to an extracorporeal membrane oxygenation (ECMO) system. ECMO was initiatedin uteroat a flow rate sufficient to support the fetus totally, the umbilical cord was occluded, and the fetuses were delivered into a warm isotonic saline bath. Breathing activity was present periodically before connection to the ECMO system and on ECMO during fetal normocapnia and normoxia. Near delivery there were no breathing movements, because all ewes were in labor. After delivering the fetuses into the warm saline bath, breathing movements remained episodic, being absent during high voltage electrocortical activity, whereas fetal Paco2remained constant. However, after 36-192 min, breathing activity became present continuously in all animals, at a time when fetal central temperature decreased. Once initiated, continuous breathing could be stopped by reducing the Paco2. We conclude that maintenance of fetal Paco2and a slow decrease in central temperature after cord occlusion delays the establishment of continuous breathing, and that the level of Paco2is important in the maintenance of breathing activity during early postnatal life.Abbreviations: ECMO,extracorporeal membrane oxygenation;HV,high voltage;LV,low voltage;ECoG,electrocortical activity;EMG,electromyogram