1. Previous studies of the ventilatory response to metabolic acidosis have usually considered only patients with arterial blood pH above 7·10. To define the response during more severe acidaemia, arterial CO2 tension and pH were measured in fifty-three episodes of diabetic ketoacidosis, including twenty-four with pH below 7·10, and ten with pH below 7·00.

2. The relation between arterial CO2 tension, and both blood pH and plasma bicarbonate concentration, in these cases with generally severe metabolic acidaemia (mean pH 7·12 ± SD 0·13), was very similar to the relations between those variables found by others in patients with less severe acidaemia, such as that due to renal failure.

3. As arterial blood hydrogen ion activity increased, arterial CO2 tension decreased inversely, reflecting well-sustained hyperventilation, even during profound acidaemia.

4. The inverse relation between arterial CO2 tension and hydrogen ion activity suggests that during metabolic acidosis, alveolar ventilation increases in direct proportion to the increased blood hydrogen ion activity.

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