ExtractFocal electrical stimulation of the medulla was undertaken in exteriorized fetal sheep and neonatal lambs, anesthetized with pentobarbital. The area from which respiratory responses were obtained was mapped using stereotaxic coordinates. Focal stimulation of the fetal medulla caused one or two inspiratory gasps. In only a few instances was the response expiratory and this appeared to be nonspecific. The area in the fetal brain stem from which respiratory responses were elicited was diffuse and extended from obex to lower pons. This region occupied a volume of about 130 mm3.In one exteriorized fetus with spontaneous, rhythmic ventilation, focal stimulation caused an apneic pause. Later when respiration ceased spontaneously, focal stimulation at the same current strength induced inspiratory gasps. In the spontaneously breathing neonatal lamb, focal stimulation of the medulla was associated with a diminution in tidal volume and an apneic pause could be produced with an increase in current strength. The apneic pause occurred at about the same current strength as that required to initiate respiration in the fetus.In the neonatal lamb the brain stem area from which a respiratory response could be obtained was less diffuse than that of the fetus and occupied a volume of about 65 mm3. This region was similar to the inspiratory center of adult sheep reported by others.SpeculationThe respiratory center of the fetus appears to be just as sensitive to focal electrical stimulation as that of the neonatal animal. However, differing ventilatory response to stimulation depending on the presence or absence of respiration suggests a different functional organization of the fetal respiratory center. This is further supported by the characteristic inspiratory response of the fetus and the diffuse area from which such a response was obtained.