A slight but real acidosis, involving a temporary decrease in alkaline reserve and increase in urinary acidity, occurs after all inhalation anaesthesias. Ketonuria is also present in 20 to 80 per cent. of cases. Local anaesthetics also produce these conditions, but less severely and less frequently.The acidosis seemed to depend on complex factors, but mainly on tissue anoxaemia produced by the upset of normal respiration and circulation during operative procedures. In the normal individual this acidosis is very slight, transient, causes no symptoms and is of no moment. It cannot be entirely prevented by previous administration of sodium bicarbonate, glucose and insulin, although high carbohydrate diet before operation minimizes the tendency to acidosis. Cases with acidosis before operation become worse afterwards and require treatment to prevent complications.The pre- and post-operative treatment of acidosis and ketosis in normals and diabetics, especially in emergency conditions, is discussed.