Movement of the chest wall and abdomen during episodes of breathing has been measured by an ultrasound technique in ten fetal lambs. 2. The fetal thoracic and abdominal walls move inwards and outwards respectively during inspiration. The position where this transition occurs is variable. 3. Movement of the lateral thoracic walls is reduced or reversed during breathing stimulated by hypercapnia while inward dorso-ventral movement increases. 4. Deep inspiratory efforts were associated with large inward then outward movements of the lateral and large inward movements of the dorso-ventral thoracic walls. 5. No relationship was found between the amplitude of the tracheal pressure deflexions and the extent of thoracic wall movements during fetal breathing.
