1. The metabolic changes following thoracic surgery in three groups of patients, (oesophageal cancer, lung cancer and hiatus hernia) have been studied. 2. Fasting levels of plasma glucose in patients with tumours of the lung or oesophagus were within the normal range and were no different from those found in patients with hiatus hernia, before operation. 3. Hyperglycaemia occurred after oesophagectomy, oesophago-gastrectomy and herniorrhapy. Operations of the lung, such as pneumonectomy or lobectomy did not lead to an immediate rise in blood sugar Level after surgery. 4. Post-operative hyperglycaemia was accompanied by the fall in the levels of plasma glucogenic amino acids. Evidence is presented in support of the idea that post-operative hyperglycaemia is the result of increased glucose production rather than the decrease in glucose utilization. 5. In contrast to the plasma insulin concentrations which remained unchanged immediately after surgery, the Levels of plasma 11-hydroxy-corticosteroids rose immediately after operation and that was accompanied by the same rise in the Levels of plasma FFA. 6. Plasma insulin concentration rose significantly and the rise was not proportional to the level of blood glucose on the second post-operative day. Since the urinary excretion of ketone bodies was also high on the same day, there was evidence of postoperative insulin resistance. 7. Elevated plasma levels of glucagon coincided with hyperglycaemia in oesophageal cancer patients but did not occur in lung cancer patients in whom there was no hyperglycaemia. 8. The plasma free tryptophan level in patients with oesophageal or lung cancer tended to be lower than in patients with hiatus hernia. Furthermore, the concentration of plasma free tryptophan rose after surgery and this rise was associated with a fall in the level of plasma total tryptophan. 9. There was no significant correlation between the level of plasma tryptophan and the rate of urinary excretion of N'-methylnicotinamide (NMN) in patients with oesophageal cancer. The significance of these findings has been discussed in relation to the metabolism of tryptophan. 10. The concentration of copper in the plasma was found to be elevated in patients with oesophageal cancer. Thoracic surgery was not associated with a consistent change in the level of plasma copper. 11. There was a transient fall in the level of plasma zinc after operation and this was associated with a similar fall in urinary excretion in hiatus hernia and oesophageal cancer. 12. Urinary levels of cyclic AMP or GMP in patients with tumours of the lung or oesophagus were no different from those found in patients with hiatus hernia. Cyclic GMP increased after surgery, and was higher in patients with malignancy than in patients with hiatus hernia. 13. Post-operative parenteral nutrition prevented the fall of plasma amino acids and led to a rise of plasma albumin. It also diminished the urinary losses of nitrogen on the second post-operative day
