Prenatal lung development requires fetal breathing movements (FBM). To investigate the dependence of FBM on feedback originating from the lung, we hypothesized that pneumonectomy stimulates FBM. Time-dated fetal sheep underwent bilateral pneumonectomy, unilateral pneumonectomy, or sham surgery at 125-130 days gestation. The incidence of FBM decreased in sham-operated fetuses at 142 days versus 130 days (p = 0.013), but was unchanged across all gestational ages in bilaterally pneumonectomized fetuses (p > or = 0.52). In unilaterally pneumonectomized fetuses, the incidence of FBM remained unchanged until 139 days and was higher than that of the bilaterally pneumonectomized fetuses at 130-136 days gestation (p < or = 0.03). The amplitude of integrated diaphragmatic electromyographic activity (integralEMG(di)) and total respiratory output (frequency of breathing x integralEMG(di)) were lower in pneumonectomized fetuses versus sham-operated fetuses at later gestational ages (p < 0.05). These decreases in integralEMG(di) and total respiratory output were most pronounced at 142 days in bilaterally pneumonectomized fetuses versus sham-operated fetuses (p = 0.006 and 0.016, respectively). Low-voltage electrocortical activity (ECoG) increased, and high-voltage ECoG decreased, in unilaterally pneumonectomized fetuses compared with sham-operated fetuses (p = 0.04). In conclusion, we provide new evidence that feedback from the fetal lung modulates the incidence and various components of phrenic nerve output, suggesting a positive feedback mechanism between FBM and lung development.