The fetal respiratory and electrocortical effects of thyrotropin-releasing hormone (TRH) administered into the lateral cerebral ventricles, have been investigated in chronically catheterized unanesthetized fetal sheep at 125–140 days of gestation. Stimulatory effects on fetal breathing movements were seen at doses as low as a lug bolus. TRH given as a 5-μg bolus followed by a 10 μg/h infusion for 2 h induced a rapid switch to significantly faster, deeper, and continuous fetal breathing movements, while the electrocorticogram remained episodic. Fetal breathing movements did not stop during hypoxia. TRH given as a 2-μg bolus followed by a 4 μg/h infusion or as a 5-μg bolus followed by a 5 μg/h infusion induced the same stimulation of FBMs, but breathing essentially remained episodic, state related and inhibited by hypoxia. As hypothermia presumably induces a surge in TRH secretion at birth it is possible that TRH has some role in the switch from fetal to postnatal breathing patterns.