SUMMARY1. In foetal lambs from 40 days gestation (0.27 of term) onwards delivered into a warm saline bath, apparently spontaneous breathing movements were present intermittently. They became deeper and more rapid with increasing age.2. In foetal lambs (from 0-66 of term) in which observations were made for many days after chronic implantation of tracheal, carotid and amniotic catheters, rapid irregular respiratory movements were present up to 40 % of the time, and brief gasps also were seen.3. The presence of these movements was unrelated to the foetal carotid blood gas values over a wide range of spontaneous variation.4. These foetal breathing movements were accompanied by comparatively small alterations of pulmonary volume recorded from a tracheal flowmeter, insufficient to clear the tracheal dead space. Occasionally a more prolonged expiration led to the outward flow of fluid.5. A description is given of sleep and wakefulness in foetal lambs from 0-78 of term.6. Rapid irregular breathing was associated with rapid eye movement sleep as seen in a warm saline bath or, in utero, as inferred from records of eye movements and electrocortical activity.7. Respiratory movements were often associated with relatively large variations in foetal heart rate, blood pressure and descending aortic blood flow.8. Rapid irregular foetal breathing was unaffected by section or blockade of the cervical vagi, but was abolished by general anaesthesia.