Diaphragmatic electromyographic activity, tracheal and amniotic fluid pressures, lung liquid flow, and carotid and jugular venous pressures were measured on eight fetal lambs who survived for periods of 9-43 days postoperatively. The fetal gestational age ranged from 98 to 113 days at operation. Respiratory center output of the fetus as indicated by electromyographic activity was modified by the following stimuli. It was suppressed by anesthesia and fetal hypoxia (Pao2 = 12 mmHg), tonically reduced by lung inflation, and stimulated by cyanide injections (150-600 mug) into the fetal jugular vein. Neuromuscular transmission to the diaphragm was blocked with d-tubocurarine (0.2-0.6 mg). These experiments indicate that central and motor pathways to the diaphragm are sufficiently mature by 101 days in the fetal sheep to permit their output to be modified by chemical and mechanical stimuli.