1. Resting forearm blood flow was increased in untreated juvenile diabetics and a slight but significant reduction took place during insulin treatment. The peak flow after ischaemia was also increased, but no difference could be demonstrated before and after insulin administration.
2. Reactive hyperaemia was found to be considerably prolonged in the diabetic patients with moderate ketosis and mild disturbances of the blood acid-base balance as compared to non-diabetics. This abnormality disappeared after insulin treatment.
3. The prolonged blood flow recovery showed a strong association with total CO2 in plasma. In subjects with non-ketotic metabolic acidosis of a degree similar to that seen in the diabetic patients no abnormality was found.
4. Urinary excretion of catecholamines was normal in the untreated juvenile diabetics. The observed alterations in the diabetics are similar to those observed by others in non-diabetics after administration of adrenaline or reserpine.