The development of the future air breathing respiratory system was investigated in eight carunclectomised fetal sheep and five control fetal animals over the last third of gestation (approximately day 100-day 145). Carunclectomy resulted in the test fetal animals being hypoxaemic, hypercapnic, hypoglycaemic and acidotic. All fetal animals were chronically instrumented with diaphragmatic electromyographic leads, fetal jugular vein and carotid artery catheters, an amniotic fluid catheter and small electrocardiographic leads placed on the chest wall. There was evidence of a significant, but small, reduced relative lung size in the carunclectomised animals by day 140 without corresponding relative changes in the weight of the brain, liver, or heart. Diaphragmatic electromyograph (EMG) was analysed in detail over the last third of gestation. In both groups of animals, the normal reduction in fetal 'breathing' rate, and the normal increase in fetal apnoea was noted with increasing periods of diaphragmatic silence as fetal apnoea was noted with increasing periods of diaphragmatic silence as gestation advanced. Linear regression analyses revealed that fetal breathing rate was significantly reduced over the last third of gestation, whilst fetal apnoea significantly increased in the carunclectomised animals. Average total integrated diaphragmatic activity was reduced prior to 120 days. This reduction was evident in the strength of each diaphragmatic contraction before 110 days. In three test and three control animals, histochemical analyses were undertaken to fibre type the muscles of the diaphragm. Fatigue resistant fibres were significantly reduced in the carunclectomised fetuses, and fibre diameters were diminished. Centroid analyses of the power spectrum of the diaphragmatic EMG did not however support the concept of increased muscle fatigue in the test animals.