Plasma Arginine Vasopressin (AVP) was measured serially in 10 patients during the first 24 hours of treatment of diabetic ketoacidosis. AVP was elevated in all cases initially, ranging from 4.0--122.0 pmol/l, the basal level in healthy hydrated subjects being 1.57 +/- 0.59 pmol/l (+/- 1 SD). The levels fell progressively during the course of treatment. While there was no evidence that the massive increases in AVP noted in this study were associated with water overload it remains to be determined whether the high levels observed have any significant metabolic or haemodynamic role in this disorder.