1. The relationship between end tidal position (ETP) and the point of lung emptying at which there is significant airways closure (CP) has been investigated before and after upper abdominal surgery in thirty-one patients.
2. A significant negative correlation between the index (ETP-CP) and the alveolar-arterial Po2 difference (A-ado2) was found.
3. Nineteen of these patients had a vagotomy and drainage operation and, in this group, there was a greater fall in ETP than in CP in the first and second postoperative days.
4. It is suggested that airway closure is a contributory factor to the known hypoxaemia following abdominal surgery.