To test the hypothesis that continuous fetal breathing could be induced by hyperoxemia alone or by hyperoxemia and umbilical cord occlusion, even in the absence of a rise in arterial PCO2 (PaCO2), we studied 18 chronically instrumented fetal sheep on 34 occasions using our window model (18). After a resting cycle (1 low-voltage followed by 1 high-voltage electrocortical activity epoch), the fetal lung was distended via an endotracheal tube using mean airway pressure of approximately cmH2O. Inspired N2, 17% O2, and 100% O2 were given to the fetus during one cycle each. While 100% O2 was given, the umbilical cord was occluded (balloon cuff).(ABSTRACT TRUNCATED AT 250 WORDS)